Complete admin APIs for SSH Keys

In gitlabhq/gitlabhq#3146, an admin API to *add* SSH keys to a specified user was added. This pull request complements this by adding a *list* and *delete* API. This way, admin users can fully manage users' SSH keys.

* `GET /users/:uid/keys` -> Get a list of a specified user's SSH keys (:new:)
* `POST /users/:id/keys` -> Create new key owned by specified user.
* `DELETE /users/:uid/keys/:id` -> Deletes key owned by a specified user. (:new:)