corrected typo in views.py
Added decision constants for occurrence form.
Improved models.

Made occurrences be calculated via generators.
Added custom delete method.
Updated month template with new link to detail view.
Corrected mistake in EventFactory
Removed HOURLY constant.
Added OccurrenceViews and urls.
Corrected utils to be able to work without end date.
Added OccurrenceForm
Added templates for occurrence views.