Replace terminate/2 with terminate/3, adding a Reason

This should have been done a *long* time ago, back when I initially
added Websocket support. This is the first part of two in improving
loop handler support with regards to socket closure.

Reason may include: {normal, shutdown} for the most normal shutdown,
{normal, timeout} for a loop handler timeout shutdown, or {error, _}
if an error occured.

test/chunked_handler.erl

@@ -2,7 +2,7 @@
 
 -module(chunked_handler).
 -behaviour(cowboy_http_handler).
--export([init/3, handle/2, terminate/2]).
+-export([init/3, handle/2, terminate/3]).
 
 init({_Transport, http}, Req, _Opts) ->
 	{ok, Req, undefined}.
@@ -13,5 +13,5 @@ handle(Req, State) ->
 	cowboy_req:chunk("works fine!", Req2),
 	{ok, Req2, State}.
 
-terminate(_Req, _State) ->
+terminate(_, _, _) ->
 	ok.