ICS Feeds do not work

While ICS Feeds can be specified in the admin for a Calendar, their events are not fetched and shown along with the local events.  This pull request improves, but doesn't completely fix, the issue.

By using the CalendarAnnouncement subclass of CalendarDateTime, which contains Title and Content attributes, we are able to mix in the SUMMARY and DESCRIPTION data, respectively.  

Changes to ICSReader are limited and somewhat hacky... parameters passed in the key (between ; and the terminating :) are lost.  Other issues also remain:

1. Recurrences do not expand
2. Timezones are not respected
3. All Day events are shown to span two days
4. Description text does not render well
  * HTML markup appears in plaintext
  * Random spaces appear between within words
5. Large ICS calendars can take a while to process, and are processed each time the Calendar is loaded.