Make exception handling in AsyncHTTPClient more customizable

Make exception handling in AsyncHTTPClient more customizable

Call handle_callback_exception() with the callback as argument
instead of hardcoding the call to logging.debug(). This way, users
can add their own exception handling code by subclassing
AsyncHTTPClient.

Default implementation for handle_callback_exception() calls that
same function on the IOLoop associated to this AsyncHTTPClient
instance, so users can handle any exceptions raised from their
callbacks just by overriding handle_callback_exception() in IOLoop.