orm mapping missing SonataMediaBundle 

I had a lot of trouble with this: for the doctrine.orm.entity_managers.default.mappings section it is said to add ApplicationSonataMediaBundle, but then, when you update the database schema, you get tables with only an 'id' field. So, after much googling I found out that you should also add SonataMediaBundle, and also your own bundles, because somehow the auto_mapping does not work anymore.
Fix missing validation constraint
Update BaseMedia.orm.xml

set context column length to 255 like in BaseGallery.orm.xml
Update BaseMedia.orm.xml
Update BaseGallery.orm.xml
Merge pull request #379 from mikemeier/patch-1

Update BaseMedia.orm.xml
Merge pull request #371 from StigOHara/patch-1

orm mapping missing SonataMediaBundle
Outdated documentation fix

Fixed outdated documentation about registering custom provider service
Fix #191
Merge branch 'sonata/master'