Rename group principal_id to id
Rename group principal_id to id

makes certain checks where we can be dealing with both users and groups more straightforward
fixes #61

Coenego/Hilary,jfederico/Hilary,simong/Hilary,nicolaasmatthijs/Hilary,timdegroote/Hilary,mrvisser/Hilary,stuartf/Hilary,jfederico/Hilary,Orodan/Hilary,nicolaasmatthijs/Hilary,ExC0d3/Hilary,simong/Hilary,mrvisser/Hilary,stuartf/Hilary,ExC0d3/Hilary,Orodan/Hilary,timdegroote/Hilary