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.
Are you more concerned with what this function does, or with how it's called? My intention was to have something like the heartbeat figure out if a connection is lost and trigger the on_close method on all the registered applications. Does that work?