Event exclusions

https://www.pivotaltracker.com/n/projects/982890/stories/78137552
add exclusions to events.
    - improves next_event_occurrence related methods to allow for exclusions    
    - adds rspec model tests to verify exclusions
some refactoring of events model...
    - changes the before and after windows for event series calculations (@@collection_time_future and @@collection_time_past) into constants.
    - clarifies name of get_next_scrum/get_next_event methods
a more concise statement would be:

```ruby
{ event: self, time: _occurences.first.start_time } if _occurences.present?
```