Filtering, groups and events: model and saving

Regarding issue https://github.com/getsentry/sentry/issues/1267
Created the eventfiltertagvalue model, and created migration for it.
Saving the new data to database is done in event_manager.py

This is the first draft with just "It runs" tests done to it. 

What do you think about the saving part. My knowledge of djangos querysets is limited and so i don't know how efficient my code really is.

I tried to be as consistent with other models as possible.
The foreign key `messagefiltervalue` refers to the id of the table with the same name. Maybe it would be better to refer to the model instead, in this case `grouptagvalue`?

I will commit the changes when I get the chance.