A more fine granular way to grant Pad access

In Etherpad:
* Each Author has his Group
* All Pads belong to Group
* Access to a Pads is given through a Group access

See: http://etherpad.org/doc/v1.4.1/#index_overview

Result:
* Possibility to access all Authors Pads if an access is given to a Pad.

Expected:
* Only and only the Pad shared can be accessed.
* If we go there, maybe wise to have smarter permission engine - can_read, can_write...

This is no simple job I guess, as it shakes the fundamentals of how the authorization works in Etherpad (http://etherpad.org/doc/v1.4.1/#index_overview).