Recurring events

As it was written in #2, this is the continuation of the implementation of the recurring events parsing.

Should be based on the [RFC 2445](http://www.ietf.org/rfc/rfc2445)

- [ ] Parse RRULES correctly
  - [ ] Check up the grammar at http://www.kanzaki.com/docs/ical/recur.html
- [ ] Handle short cancelled recurred event (Google)
- [ ] Handle recurring events correctly (Google)