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
We should add documentation regarding the fact that this can be a very memory intensive and slow performing operation when a cache loader is used and advise using SKIP_CACHE_LOADER when possible.  The same for removeGroup.