Fix disconnect bugs

Right now disconnect is not working as expected, so stomp.py receiver
loops are not stopped. I'm using the Connection.connected property that
seems to be bugged now and thus, I should be using the
Connection.is_connected method instead.

I was also calling to Connection.stop manually, however
Connection.disconnect already calls it.
Merge pull request #12 from rafaduran/bug/disconnect_does_not_work

Disconnect bug