Changes towards enabling search to be extended by Media bundle

This PR:

- allows SuluSearch metadata objects to be modified by event listeners.
- prepends a configuration to MassiveSearchBundle to use a custom Factory object so that we can create our own (Sulu) classes.

This is required to enable bundles such as MediaBUndle to remain uncoupled from SearchBundle.
This is "unfortunate". I would rather not talk about "thumbnails" in this class. (and it should probably be renamed to "images"), but a more flexible solution would be another days work. Have created an issue instead: https://github.com/massiveart/MassiveSearchBundle/issues/9