Update erlang.mk
Rename 'shutdown' close reason and tuples to 'stop'

The 'shutdown' atom has a specific meaning inside OTP. We are
instead going to use 'stop' which is pretty much the equivalent
of what we actually do. 'shutdown' is now reserved for future
special processes implementation.
Rename 'halt' to 'stop' for better consistency

Now everywhere in Cowboy when we want to stop something we return
a 'stop' tuple instead of one of the many choices depending on
context that we had before.

This particular change affects middlewares, sub protocols and
REST handlers which were using 'halt' to stop processing.
Add an RFC7230 HTTP/1.1 server specification document

This document lists all rules that Cowboy HTTP server will follow
after careful analysis of the RFC7230.

Do note that not all these rules are implemented or tested yet,
and that at the time of commit there may be rules that Cowboy will
ultimately not implement (for various reasons including performance
or leaving certain aspects up to the user code).
Add a specs document for RFC6585

Only a few status codes. Comments in the previous commit apply.
Docs clarification when returning a list of ws frames