add a controller to ask for ssh access

- Add a Controller to allow queries, which
  ask for access on a Repository by git.
Implement Querying for SSH-Permissions.
modify gitshell to query our sshaccess controller.
remove user from repository

- user is not tied to the database model
  Actually users are tied to a repository
  through permissions.
- Furthermore it should be revisited if
  the attr_accessor are really necessary.