Add APIs for Category, Collection and Tag models

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**Warning:** This PR needs to be merged before: https://github.com/sonata-project/SonataClassificationBundle/pull/21

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I've added the following methods:

![capture decran 2014-03-05 a 16 37 52](https://f.cloud.github.com/assets/103900/2334819/2d84572c-a47d-11e3-9817-136a449cf535.png)