Add guard to project

Autoruns tests for you. To use, run `bundle exec guard`.
Make tests pass

test_todo.rb require was failing. Fixed by adding File.expand_path. This may be a 1.8.7 vs 1.9.3 difference in require. Also, had a failing test that seemed to work well except the SEQUENCE output was off. `SEQUENCE;CUSTOM=yours;2` instead of `SEQUENCE:2`. Not sure where it came from so disabled the test for now.
Add occurrences_between with support for basic daily recurrence

Including exception dates! See tests for more detail.

Use like this:

```ruby
calendars = Icalendar.parse(File.read("some.ics"))
my_event = calendars.first.events.first
occurrences = my_event.occurrences_between(Time.now, Time.now + 10.days)
occurrences.first.start_time
# => 2014-01-13 00:00:00 -0800
occurrences.first.end_time
# => 2014-01-13 00:00:00 -0800
```
Add tests for complex intervals with daily recurrence
Add support for weekly occurrences
Add yearly and more daily recurrence case handling
Move recurrence fixtures out of test files + small tweaks
Add until/count support
Fix recurrence rule bugs