Integrating images into search

This PR extends the MassiveSearch bundle and maps the IMAGE URL onto the search result document if the subject is a SuluMedia image.

## Todo

- [x] Reviewed by Dan
- [ ] Reviewed by others ..
We can use `getEnvParameters` instead of this.

````php
protected fucntion getEnvParameters()
{
    return array('sulu.context' => 'asd'); // should do an array merge with parent really
}
````