REST services using doctrine_odm

This PR contains my work on making doctrine ODM based REST services.

Please have a look at the tests in ``src/Graviton/CoreBundle/Tests`` and ``src/Graviton/TaxonomyBundle/Tests`` to see what has already been implemented.

If you run ``phpunit -c app/`` you can see that there are still some tests that need implementing. Nevertheless I propose merging this to develop so we can start building on it.

There is also still some need for cleaning dead code (mainly in ``src/Graviton/RestBundle/Model``). I think most of it can be cleaned and refactored after we decide how we will be using doctrine ORM in the future.

I'll probably end up doing some small refactoring over the next couple of days. Don't expect large changes since I consider this mostly ready for review. Among other things I need to write tests to get the code coverage up to over 80%.