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
https://issues.jboss.org/browse/ISPN-4452
@pruivo Please remove the snapshot wording here as well.
Other than this, I'm ok with the PR.