Add support for deleting user keys to SSHClient.
Review Request #3461 — Created Oct. 27, 2012 and submitted
Add support for deleting user keys to SSHClient. This allows SSHClient to delete a user key. It provides a delete_user_key function on SSHClient and SSHStorage. This can be used to provide a UI for deleting a user key. This UI doesn't exist in Review Board yet, but will be easy to wire up at some point.
Unit tests pass.