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.
This is required in order to handle system messages correctly. The very minimal is done with the `dbg` object: it is passed around but no debug events are generated and it is never passed to the middlewares. If we do not pass it around we create unpredictable handling of system messages and then the implementation would be broken. Therefore I am not keen to remove this minimal handling. The minimal handling is reasonably tested in the test suites.