According to the spec, EXDATE properties can have multiple values. However, many
popular calendar client don't support that, and only accept iCal feeds which
use multiple separate EXDATE properties. That is spec-compliant, though, so I
refactored Date iCal to do that for EXDATEs and RDATEs.

The previous fix that I applied for issue #2070883 was the complete wrong way to
go about fixing that problem, because it introduced a new, much more sinister
issue: the UID for an event would change every time the iCal feed got downloaded.

According to the spec, the UID is supposed to be persistent, so I re-engineered
the algoithm that creates UIDs for the Fields plugin so it will create the same
UIDs as the Entity plugin does.

Issue [#2046935]: Added support for "long text" and "long text and summary" for the LOCATION and SUMMARY fields.

date_repeat_split_rrule() calls functions from other .inc files, but doesn't
directly load those files. So I made Date iCal load them before calling it.

date_ical_format_text() was going way overboard on it's attempts
to make text iCal compliant. I got rid of all the escaping, and also
all the extra endlines which the drupal function was adding at 80-char
boundries.

I added the summary_field option to option_definition, which should
fix several (surprisingly unreported) errors related to Features and first-time
setup.

Added the view object as a parameter for theme_date_ical_icon, which gives
additional context for any module that would like to override that theme.

Issues [#1931020] and [#1935820]: Allow custom fields for summary. Fix repeating events causing duplicates. Add @file to DateIcalFeedsParser.inc

- Issue #1529506, Added optional webcal:// support.
- Issue #1890736, Added ability to exclude the X-WR-CALNAME property.
- Issue #1565410, Sites with "Clean URLs" disabled can now download feeds normally.
- Several minor bigfixes, and cleanup of the iCal icon theming.

Includes patches from issues 1895396 and 1462646.
Updated the README and the CHANGELOG.

If the user specified a date field in the Show settings which didn't exist in one or more of the entities which passed through the View, the rendering process would crash. This should no longer be the case.