Relax restrictions on HTTP methods in WebSocketHandler.

Methods like set_status are now disallowed once the websocket handshake
has begun, but may be used before then.  This applies to application
overrides of prepare() and to WebSocketHandler.get's internal error
handling.

Closes #1065.
Rename HTTP{In,Out}putException to HTTP{In,Out}putError.

This follows the standard python naming convention for exceptions.
Introduce IOLoop.spawn_callback.

This makes it easier and more discoverable to run a callback without
stack_context.

Closes #867.
Fix a leak in AsyncHTTPClient shutdown.

Clients created without force_instance=True were never getting
GC'd because we were using the wrong reference for the async_clients
instance cache.

To guard against future errors, failure to remove an instance
from the cache when we expected to now raises an error instead of
failing silently.

Closes #1079.
Introduce IOLoop.call_later and call_at.

call_later is a less-verbose alternative to add_timeout with a
timedelta; call_at exists for symmetry.  Both are named after
methods on the asyncio event loop, although there are small
variations (we support both args and kwargs while asyncio only supports
args; we use remove_timeout(handle) instead of handle.cancel()).

Closes #1049.
Add new exception tornado.web.Finish to quietly end a request.

This allows error pages to be generated inline with the main code
instead of in write_error and is friendlier to generating error pages
from library code.

Closes #1064.
Release note updates.
Add pypy3 to tox.ini.
Remove workarounds for ancient versions of libcurl.
Remove support for the old get_error_html function.
Increase the test timeout on travis.
Update tests after the removal of get_error_html.
Edit comments to discourage creation of new OutputTransforms.
Drop support for the draft76 version of WebSockets.

Browsers that only support draft76 are now less common than those
that do not support websockets at all, so applications should
use their non-websocket workarounds for these browsers.
Update docs for deprecations and removals.
Set version number to 4.0b1