Add groups write APIs

I've added write APIs for user groups:

![capture decran 2014-03-17 a 15 34 51](https://f.cloud.github.com/assets/103900/2436908/4801132a-ade1-11e3-836d-6178eb0cd2e8.png)

Behat tests has been updated in sandbox (PR https://github.com/sonata-project/sandbox/pull/326)