Cleanup API to connect to services

[Natalia Bidart]

We need to get rid of the "subscribe_to_signals" and "unsubscribe_to_signals", otherwise we get traces like this when a client that connected to a service dies or disappears:

2011-07-06 15:19:38,082:82.9999446869 - ubuntu_sso.main.windows - WARNING - Could not emit signal due to

[Failure instance: Traceback (failure with no frames):
<class 'twisted.spread.pb.PBConnectionLost'>: [Failure instance: Traceback (failure with no frames): <class 'twisted.internet.error.ConnectionDone'>: Connection was closed cleanly.]
]

*--- Failure #6 ---
Failure: twisted.spread.pb.PBConnectionLost: [Failure instance: Traceback (failure with no frames): <class 'twisted.internet.error.ConnectionDone'>: Connection was closed cleanly.
]
*--- End of Failure #6 ---
2011-07-06 15:19:38,091:91.0000801086 - ubuntu_sso.main.windows - WARNING - Traceback is:
None