#888 - Removed check in group.create for existing ID

Also included a validator check for valid roles and principal IDs
when creating a group and moved the Cassandra logic into the DAO
#888 - cleanup

* Cleaned up principals constants
* Removed unused `otherValues` parameter from the createGroup function
#888 - Cleaned up jsdoc
Merge branch '888-remove-group-id-check' of http://github.com/simong/Hilary into 888-remove-group-id-check
Documentation clean-up