Websockets, Streaming content, Gunicorn Worker

To see the exhaustive list of pull requests included in this release see:
https://github.com/channelcat/sanic/milestone/6?closed=1

Highlights

• Allow a socket to be passed even when not multiprocessing #506 (Thanks to @zenixls2)
• Websockets! #469 (Thanks to @miguelgrinberg)
• Streaming content handlers #473 (Thanks to @subyraman)
• A gunicorn worker! #545 (Thanks to @messense)
• Setting configurations through environment variables #573
• Adding a way to pass in a custom request class #622 (Thanks to @aryeh)