Added a Solarium Data Fixture Loader Command

This adds fixture loading capabilities to the bundle. Basically only a command and a couple of services are needed. That way, it's easy to load Solr data fixtures in a Symfony application. The feature is merged in Solarium, however, not in master (that's why I have updated the composer file, this should be changed back before this is merged. 

I will add Alice related capabilities that use this in the AliceBundle.