New routes

This PR does a lot of things:

- Move to django REST framework 3.0
- Ability to create users from the Web-browsable API as admin
- Hyperlink models for easier access on the Web-browsable API
- New routes
 - `readings/aggregate/` -> Aggregate readings by hour
 - `readings/?users=arduino1,arduino2` -> Filter by 1+ devices
 - `readings/?created=2014-12-17` -> Filter by start date
- Data aggregation and filtering happens in the Postgres database using QuerySets, to ensure consistency and speed