Bumps version to 0.21a1.
http.py: Refactors chunked transfer logic into ChunkedTransferStreamReader.
extended.py: Makes write a coroutine.
http.py: Removes unneeded use of Task.
engine,http: Adds the worker futures to the IOLoop to avoid losing them.

Will this finally fix chfoo/wpull#27 ?
Adds proxy module.
Fixes travis config for installing pycurl.
Fixes travis config with sudo for pycurl install.
Makes TestProxy tests optional for now.
proxy_test.py: Moves tornado curl_httpclient import to later.
Splits http module into http submodules.

Splits into modules http.client, http.connection, http.request,
http.util. Moves web module into http.web.
Updates backport.conf for http package.
http/client.py: Allows Recorders on initializer and fetch method.
proxy.py: Adds ProgressRecorder on debug runner.
changelog: Updates recent changes.
engine,http.py: Invokes the worker futures so errors are logged.
proxy.py: Invokes the future so errors will be logged.
Adds phantomjs module.
phantomjs.py: Fixes to use tornado.gen.Return.
Fixes docstring references to http module.