Fix #102: Don't connect twice.

This corrects an old problem and also protects against it in the future:

- ui.js only attaches the connect command to #fakeLoginForm:submit,
  not to the enter key in its text fields. This way, the command will not
  be executed twice.
- xmpp.newConnection() will explicitly disconnect before
  replacing the connection object. xmpp.disconnect() is safe
  to use regardless of connection status.