Implement categories

Hello,

I implemented event categories, as it was present in the v1 of refinerycms-calendar engine, but missing in v2, and I need this feature back for a client project.

It is implemented as a simple many-to-may (HABTM) relationship between events and categories, with proper spec coverage. All original and added specs are passing.

If you accept this feature, I'll continue to improve the implementation (better locales support & other stuff I can improve) and make according pull requests in the following days/weeks.

Thank you,
Daniel