ISPN-3981 Enhance grouping API to support group manipulation methods

* First implementation. Added getGroup() and removeGroup() methods

This is the base implementation. Added methods to return and remove the keys from a group. This may be extended to support other functionality.

https://issues.jboss.org/browse/ISPN-3981
I could say it isn't a bug in DummyStore.  These flags from my understanding are more of a optimization if possible kinda thing.  This way the store can use them if needed, but not all stores can do an optimization.  Since in the case of the DummyStore it already has the value in memory so there is no harm in always returning it.  Maybe @mmarkus can chime in on his thoughts.