Fixed bug for anonymous user accessing pad, fixed groupAttribute parameter usage and added displayNameAttribute parameter

Hello,

For my usage, I made some fixed and improvement that I propose here : 
- I had a bug when anonymous user access to a pad (displayName unavailable)
- I fixed usage of parameter groupAttribute witch is currently simply ignored ('member' attribute is hard-coded)
- I added optional displayNameAttribute parameter to permit to choose attribute used as name ('cn' attribute is actualy hard-coded and leave as default).

Thank's for this useful plugin !

Benjamin