Add Post & Comment POST/PUT/DELETE API methods

I've added `POST`, `PUT` and `DELETE` methods for posts and comments:

![capture decran 2014-02-26 a 12 37 11](https://f.cloud.github.com/assets/103900/2269476/6f18520e-9eda-11e3-94e1-cbcb048b89b5.png)

This PR should be merged after this one: https://github.com/sonata-project/SonataCoreBundle/pull/45