SMAP db can accommodate multiple survey distributions

- Added `Distribution` model
- Survey now **has many** distributions
- Distribution **belong to** surveys
- Distribution **has many** SurveyContact
- SurveyContact **belongs to** Distribution
- SurveyContact **belongs to** Contact

##### Beware

> I haven't written a schema migration, so to get this working, just delete the app from your phone / genymotion device

See https://trello.com/c/1iArnzQV