added support for JMS Serializer GroupsExclusionStrategy

A proposal to support the GroupsExclusionStrategy of the JMS Serializer.

From the updated readme in this PR:

> When using a class with JMS Serializer metadata, you can use exclude groups by using this syntax: `input="Acme\YourBundle\Entity\User@update,public"`. In this case the groups 'update' and 'public' are used.