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(
    product='Firefox',
    versions=['19.0.2', '20.0b5', '21.0a2'],
    from_date=datetime.date(2013,2,12),
)
```

Also, if you miss to supply any required parameters you'll get a 200 error back with the error. Or, if you mess up the expected value you get an error too, like this: http://cl.ly/O4AQ

I'm going to later explode bug 855485 into new bugs about:

* automatically generating HTML documentation
* blacklisting PII (personal identifiable information) by key(s)
* add a rate limiter so that it's not possible to DDOS the API