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
not quite.  I think that there are cases when the repeat_ends_on field is filled in, but repeat_ends has been changed to false.  Similarly, there are cases when the event was repeating then is no longer repeating.  This is what repeating_and_ends? checks for.  
It might not be the minimum # of lines, but I think that it's pretty clear what's going on in the logic.