implement organizations

[poblahblahblah]

It would be nice if there was a concept of "organizations" within ringring (similar to how github does it, actually).

Each org could have a separate URL for twilio integration, and they would contain their own oncall calendars and users.

Operations and Engineering, for example, would be two separate orgs. Under Operations there would be Application Support, Linux, Windows, Networking, Data Center. Under Engineering there would be whatever various teams that support Engineering.

Users can be apart of multiple orgs and calendars, their vacation would apply to everything they are a member of.