Added initial support for epoll

I understand that one of the primary goals of waitress is to maintain compatibility with windows but I think the polling mechanism could use the appropriate mechanisms when available based on the running platform.

This allows the user to choose which poller they want and as long as they choose a supported on on their platform everything is fine.

The asyncore_epoll file is a modified patch from http://bugs.python.org/issue6692 to be 100% Python3.3 compatible and fixed minor bugs with the exception handling.