When a function on the IOLoop returns a Future, log its exception.
Add ETIMEDOUT to the list of relatively benign unclean-close errnos.
Remove async_callback wrapper functions, which have been obsolete since 1.1.

Closes #283.
Improve callback scheduling.

Collect all timeouts to be run before running any of them; this
prevents starvation when a slow callback reschedules itself.

Call time() again before setting poll_timeout to avoid scheduling
anomalies with slow callbacks.

Closes #947.
Document AsyncHTTPClient's defaults argument.

Closes #762.
Add clearer message for fetch() on closed AsyncHTTPClient.

Closes #866.
Document that IOStream.connect blocks on DNS and recommend TCPClient instead.

Closes #861.