User-machine permissions table

Adds the following `/users/` endpoints:

   * `GET /users/:username/machines`
   * `GET /users/:username/machines/:hostname`
   * `PUT /users/:username/machines/:hostname`
   * `DELETE  /users/:username/machines/:hostname`

Adds the following `/machines/` endpoints:

   * `GET /machines/:hostname/users`
   * `GET /machines/:hostname/users/:username`
   * `PUT /machines/:hostname/users/:username`
   * `DELETE /machines/:hostname/users/:username`

And the `permissions` table having `user_id` and `machine_id` as columns.