add a check for the station id to Network.get_response()

obspy.station.network.Network.get_response() doesn't currently take the station id contained in seed_id into account. Instead it collects all responses that match the given channel. This results in a warning "Found more than one matching response. Returning first." and the wrong response being attached to a trace when used with Trace.attach_response(), if the corresponding inventory contains more than one station.

Sorry, for not providing a test case... however, there is no test_network.py yet and I didn't feel like spending too much time on it now ... :-)