Close socket on disconnect

[elParaguayo]

As per issue #137, sockets are not closed when the bus is disconnected. This results in users receiving warnings about unclosed sockets.

This PR adds a call to the socket's close method during disconnect.

Closes #137

[elParaguayo]

Not sure why test_tcp_connection_with_forwarding fails as it doesn't call bus.disconnect which is the only code I've changed.