Use custom logger in _ForwardServer

[sweenu]

It's problematic that we're doing create_logger() hidden in the __init__ of _ForwardServer because it modifies the custom logger that we might be passing when creating an SSHTunnelForwarder by adding a console handler.
We do not always want a console handler.

[coveralls]

Coverage increased (+0.3%) to 91.754% when pulling 1d054c3 on sweenu:use_custom_logger into 58a46ef on pahaz:master.

[pahaz]

Welcome to the CONTRIBUTOR list: d2160ff