Support read-only access to repositories

Regarding the repository.Repository struct:

* `Repository.Users`: users that with write access
* `Repository.ReadOnlyUsers`: users with read-only access

Implementation notes:

* When creating a new repository specify which users should be
  allowed read access
* Allow specifying if read-only access when adding user to repo
* When removing user from repo, remove from both write and read
collections of users

Please review, comment, suggest and advise! :wink: 
Well, these were the original guidelines for this feature:

 * Support namespaces for repositories
 * When creating a new repository specify which users should be allowed read access
 * Allow specifying if read-only access when adding user to repo
 * When removing user from repo, remove from both write and read collections of users

That's why that method is somehow *surplus* – there's no route for it as of yet.