Adds tests for d3.svg.diagonal

A mousemove event was being triggered on mobile Safari causing the brush
to jump briefly on touchstart, e.g. when viewing:

    http://bl.ocks.org/mbostock/raw/1667367/

The new d3_event_dragSuppress method serves a similar purpose to the previous
d3_event_userSelect, except it generalizes to prevent all undesired browser
default behaviors when dragging. In addition to text selection, this includes
dragging (of links and images) and clicks (on links). Suppressing click events
is optional and can be controlled by the caller.

Also, disable buffering to discard samples; all samples are retained.

Projections will typically exhibit less curvature at larger scales, with
geodesics becoming closer and closer to straight lines.  Using a minimum
angular distance for resampling therefore makes more sense than using an
arbitrary scale-dependent threshold.

It's not clear whether this new threshold should depend on the precision
parameter, or be configurable separately.

Rather than compute the distance from the start to the new middle, use the
already computed distance from the start to the end. Also, we no longer need the
additional check that the middle is close to either the start or the end because
this is covered by the minimum sample distance check.

Rather than return NaN, this change restores the previous undefined behavior
when an unorderable value is used for bisection. After discussing with Jason, I
think this is more consistent given that d3.bisect also requires that the input
array is sortable, and therefore only contains orderable values. Thus, it is the
responsibility of the caller to check that the search value is defined before
doing bisection, and so this commit fixes the threshold scale rather than the
bisect method. (Also, this is consistent with the existing quantile scale.)

Since the quantile scale allows arbitrary values in the range, there’s no reason
the return value for unorderable input needs to be a number.

In addition to NaN, which is not equal to itself, you can have objects that are
not orderable due to defined valueOf functions which return NaN. For example:

  var o = new Number(NaN);

Here, o == o is true, but o <= o is false. Therefore it was possible for d3.min,
d3.max and d3.extent to observe these non-orderable values rather than ignore
them as intended. The fix is to check !(o <= o) rather than o == o.

This commit also fixes d3.bisect when the search value is non-orderable.
Previously, bisectLeft returned lo for non-orderable values, and bisectRight
returned hi. However, if the search value is non-orderable, then this return
value does not satisfy the conditions of bisection. This commit changes the
bisection methods to return NaN when the search value is non-orderable.

As a side-effect, the fix to d3.bisect now causes d3.scale.threshold to return
undefined when passed a non-orderable value, such as undefined. (Previously, a
threshold scale would return the highest value in the range, because it used
bisectRight internally.)

Similarly, if there are no scales, setting a clamp does nothing.

For consistency with brush.extent.

The previous implementation depended on CSS3 transforms being vendor-prefixed,
and would assume no vendor prefix if the browser supports transforms natively.
Now we detect the vendor prefix on a case-by-case prefix, rather than assuming a
single global prefix.

Fixes #857. By default, the radius of each circle in the pack layout is derived
from the square root of the node’s value, which is then scaled according to the
bounding box of all circles and the layout’s size. By specifying a radius
function, you can now explictly control the radius, disabling automatic scaling.

The radius function takes the value as input and returns the corresponding
radius. If no radius function is specified, automatic scaling is enabled; this
is the default. Conveniently, d3.scale.sqrt can be used as a radius function.

The use of preventDefault while dragging or zooming was preventing
desirable defaults from being triggered.  Using user-select: none is a
less intrusive way to prevent text from being selected.

If user-select or a vendor-prefixed user-select is not supported, we
suppress the selectstart event instead (for IE9).

Note that Firefox has a bug where absolutely-positioned elements do not
inherit the -moz-user-select style from the body element, so users may
wish to additionally set -moz-user-select: none on absolutely-positioned
elements, at least until this bug is fixed:

  https://bugzilla.mozilla.org/show_bug.cgi?id=648624

Fixes #1288, #1099.

It’s not strictly geo-specific, so math seems like a reasonable place for it.