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 this is supposed to be true, false.  Otherwise the value could be null depending on the store implementation.  Actually thinking about this do we support if the user specified IGNORE_RETURN_VALUES on the cache before invoking getGroup?  I wonder if we want to not return the values then.