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
you're right once again :)

about the flag, he getGroup only respects the SKIP_CACHE_LOAD. I think for this release (alpha5) it is ok. later I can add support for other flags.