SG-iCalendar: Remove limit for recurrence if event has no limit.

SG-iCalendar used to limit events that recur forever until the next three
years.  Since we're using SG-iCalendar as a parser, we don't want any
restrictions.

See #8.
First pass at recurring events support.

This commmit:
- Introduces get_recurrence_data() method - This method parses iCal
  recurring event data to the Events Calendar PRO's recurrence spec.
- Introduces get_recurrence_by_day_data() method - Helper method to parse
  the day data, which is both used when parsing recurring months and years.

Note: Events Calendar PRO does not support super-advanced recurring events
that have weird conditions.  (View latter examples @
http://www.kanzaki.com/docs/ical/rrule.html#example.)  It does support most
simple recurring events, which should handle the majority of use-cases.

FYI, Events Calendar's method at handling recurring events sucks.  Each
event is a separate WordPress post.  This is super-bloat for recurring
events that do not have an end date!  Event Organiser is a better plugin.

See #8.