Fixed hot loop in asyncore connection.
Problem was that we were unconditionally registering as writable, causing the select call in asyncore to never ever block.
Fixed by overriding writable dispatcher subclass.
Also added timeout to select call so that the loop is responsive to user input. In Unix more elegant solutions are available, but in Windows where select() is provided by Winsock we have to resort to this. Note Twisted does the same thing!