Add support for multi-day events.
'end_date' for existing events is copied from each event's date.
copy Event.date to Event.end_date if an end_date is not provided
Removed generic field from migration. Closes #18.
Merge branch 'master' of https://github.com/jantoniomartin/mezzanine-events

Conflicts:
	mezzanine_events/migrations/0001_initial.py