Fix gunicorn worker, Update token attribute, etc.

To see the exhaustive list of pull requests included in this release see:
https://github.com/channelcat/sanic/milestone/8?closed=1

Highlights

• New contributing rules are now present for all incoming PR's
• Now allow unknown status codes per #643 (Thanks to @aryeh)
• Fix absolute path bug for static files #647
• Fix signals being shared amongst multiple Sanic apps #652 (Thanks to @38elements)
• Fix duplicate signal settings for gunicorn worker #655
• Update token attribute to function with/without Token: prefix #656
• Allow disabling of keep-alive functionality through config/environment variable. #646