Allow overriding groups when descending into referenced objects.
Update reference documentation.
Fix error with missing MetadataStack in GroupsExclusionStrategy.

The error was triggered by Nelmio\ApiDocBundle, since it calls the
exclusion strategy without a MetadataStack present.
Merge branch 'recursion-groups'
Add support for using the discriminator column as XML attribute
Merge branch 'xml-discriminatorattribute'
Merge remote-tracking branch 'andreasferber/master'