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 think we have a bug in DummyStore... it is not respecting the fetchValue and fetchMetadata, otherwise I was able to detect this bug... Good eye btw :)