on_socket_close()

Addresses #55 

Added a no-op applications can override to take action against a WebSocket close. 

PEP8 compliant.

Tested by adding on_socket_close() to notes application.
I actually think something like a on_connection_lost or on_connection_dropped would be more appropriate than on_close. I feel it's more representative and at the same time can be used to handle client drops due to network errors