Allow admins to transfer cookbook ownership

Implement the ability for Supermarket admins to transfer the ownership
of a cookbook to any other user.
Add eligible for attribute to collaborators endpoint

Depending on the context different collaborators should be returned. So
this adds an optional parameter to the collaborators endpoint
(eligible_for) which currently exepcts either collaboration or ownership
and returns the corresponding potential collaborators. If no
eligible_for parameter is supplied no users are returned.
Merge pull request #421 from opscode/admins-transfer-ownership

Allow admins to transfer cookbook ownership