Add support for multi-day events.

I have changed the Event model so that it has now a 'date' field and an 'end_date' field. Both of them are required and for a single day event they should just have the same value. For existing events 'end_date' is copied from each event's date.

Of course, I have updated all the affected code, and I have run the unit tests.

One caveat: I had to comment the 'keywords' field in the initial migration, because South threw an exception and suggested this solution (see this commit in mezzanine: https://github.com/stephenmcd/mezzanine/commit/2ceaeb685e314892ef50fa3a71e691c2a87e34c7).