Nix tastypie for django-rest-framework

This redoes the api using django-rest-framework.

I changed bits of the api which I think warranted making it a v2. I
have no interest in supporting the old one, so I nixed it. :(

Changes in the new API:

1. language is now a required field
2. no longer use api urls for category, tag and speaker--we use title,
   tag and name (respectively)
3. slug is a read-only field now

Fixes #181.
Freeze base requirements; fix reqs

Some of the requirements were kind of out of whack. This fixes that
and freezes them on specific versions.
Update CHANGELOG
Update docs; nix last vestiges of tastypie

This updates the API docs to the new v2 API.

Removes last vestiges of the tastypie bits.