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
Okay I think this is okay, but there is a weird interaction going on here since this is a List.  If you had multiple activations or even an activation after you read the value from the store, then you would have multiple values in this List.  I _think_ the value would be the same in all cases.  The only case that is a bit weird is if you somehow had multiple activations you could have a different value.

But I am thinking we don't really care, because either value is correct while doing the operation since there is no guarantee to see the latest.  I just wanted to point it out in case if you can think of anything else related to it.