add exclusions to events.
some refactoring of events model...
- changes the before and after windows for event series calculations into constants.
- clarifies name of get_next_scrum/get_next_event methods
- improves next_event_occurrence related methods to allow for exclusions
undo changes to gemfile
add database migration
revert schema.db.  I didn't mean to add it to the PR.
adds test for serialization of exclusions
And a good thing, because for some reason, self.exclusions is needed to access the member variable inside of the event methods.
Per Thomas' request, change Event#next_scrum_occcurrence to Event#next_occurrence(event_type)
added tests for next_event_occurrence_with_time
refactorings per Bryan's advice.
added byebug gem back (it was removed by mistake)
fixes self.next_occurrence refacto.  It was compacting in the wrong place.  now it compacts the result of map, which was the original intent.
restores gemfile
Merge pull request #526 from jon-engelbert/event_exclusions

Event exclusions