Fix some libphutil Calendar document bugs

Summary:
Ref T10747.

• Default to UTC if a datetime has no explicit timezone.
• Fix a bug with escape detection that could incorrectly trigger for escaped commas.
• Remove some unused properties (now properites of the RRULE itself).

Test Plan: See next diff.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16698