D3 now supports Sizzle, preferring Sizzle to the native Selectors API if Sizzle
is available. Sizzle internally uses the native Selectors API and thus this
should have minimal performance implications; also, it allows you to use Sizzle
proprietary extensions such as ":first".

This commit also restricts the definition of the enter selection so that only
append and insert operations are defined. The other operations were generally
unsupported anyway, and it cleans up the code to have separate implementations
for insert and append. (I might enable additional operations in the future, such
as `filter`, `sort` and `each`, but this seems like a reasonable first pass.)