Docs, CodingStandards & add to service for MediaCreatorService

There are 2 small breakingchanges. The constants have been fully uppercased and the getEntityManager function was removed (seemeed useless and a silly way to get to the EntityManager)

I noticed the MediaCreatorService is used in https://github.com/Kunstmaan/KunstmaanGeneratorBundle/compare/add_homepage_slider?expand=1

If this gets merged it'll have to be updated as well.