Add basic support for system messages

This changes performs the first step of #750, a simplified version with much improved testing. All system messages are handled by `cowboy_protocol` or `cowboy_middleware`. Middlewares, sub_protocols (and handlers) can not change how system messages are handled.

To have `cowboy_middleware` handle a system message a middleware (or sub_protocol) returns:
```erlang
{system, From, Msg, Module, Req, State}
```
Once the system message(s) have been handled control is passed back to the middleware via a `cowboy_sys` callback. Either to continue processing as before:
```erlang
sys_continue(Req, #state{env=Env}) ->
    {ok, Req, Env}.
```
Or to terminate with reason `Reason`:
```erlang
sys_terminate(Reason, _Req, _State) ->
    exit(Reason).
```

For more information please see the commit message or the additions to the guide.

Help is required to test handling of system messages when a websocket handler receives the message in the  `websocket_payload_loop` receive clause.
Ah yes. I made a mistake here as it has already been done[1]. Perhaps I intended to remove that linked line but in any case I will delete it here as it will create a double lookup of the receive queue in the default case.

[1] https://github.com/ninenines/cowboy/pull/754/files#diff-777c459c78111ff8fa915e7bcc366dfbL468