Refactor and add missing units tests for page & blocks APIs

I've refactored `SonataPageBundle` APIs to have a `BlockController` in addition to `PageController` for methods that only interacts on blocks.

![capture decran 2014-03-14 a 14 36 27](https://f.cloud.github.com/assets/103900/2421128/d4c87016-ab7d-11e3-9c80-f99da93529aa.png)

I've also added missing units tests, Behat tests will be added to sandbox.