Convert RatingResource and RatingFlagResource to DRF (bug 910570, bug 910571)

https://bugzilla.mozilla.org/show_bug.cgi?id=910571
https://bugzilla.mozilla.org/show_bug.cgi?id=910571

Yes, I know this is a huge diff.
![sorry](https://f.cloud.github.com/assets/187006/1541324/70641d94-4d33-11e3-824d-a454f1446f6b.gif)

Please look at the FIXMEs, I'd like a discussion on those. In particular, I'm not sure what's the best approach regarding pagination: tastypie style, with `offset` instead of `page`, seems to add nothing of value, and is incompatible with all other Paginator classes that inherit from django's. I've kept backwards-compatibility in a hackish way, but maybe we could get away with changing it to the standard django-style, even if it's a backwards-incompatible change ? This is going to be a problem when converting `SearchResource` as well.