Bug855485 public rest api

@rhelmer @lonnen r?

This exposes virtually every model class in crashstats/crashstats/models.py so that you can reach them as a restful api. For example:
`/api/CrashesPerAdu/?product=Firefox&versions=19.0.2&versions=20.0b5&versions=21.0a2&from_date=2013-02-12`

It's the equivalent of doing:
```python
api = models.CrashesPerAdu()
result = api.get(
)
```