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 the behaviour used by `gen_server` (and all other OTP behaviours). I believe that offering the choice to handler is wrong because if a handler ignores this message (does not terminate) it is going against the guarantees of OTP supervision trees by breaking the shutdown protocol[1][2]. Note that ignoring the message (due to a catch all clause) or a `function_clause` error would be the default behaviour - which is very undesirable.

[1] http://www.erlang.org/documentation/doc-4.9.1/doc/design_principles/spec_proc.html#7.2
[2] http://www.erlang.org/documentation/doc-4.9.1/doc/design_principles/sup_princ.html#shutdown