Improve handler interface and documentation

This change simplifies a little more the sub protocols mechanism.
Aliases have been removed. The renaming of loop handlers as long
polling handlers has been reverted.

Plain HTTP handlers now simply do their work in the init/2
callback. There is no specific code for them.

Loop handlers now follow the same return value as Websocket,
they use ok to continue and shutdown to stop.

Terminate reasons for all handler types have been documented.
The terminate callback is now appropriately called in all cases
(or should be).

Behaviors for all handler types have been moved in the module
that implement them. This means that cowboy_handler replaces
the cowboy_http_handler behavior, and similarly cowboy_loop
replaces cowboy_loop_handler, cowboy_websocket replaces
cowboy_websocket_handler. Finally cowboy_rest now has the
start of a behavior in it and will have the full list of
optional callbacks defined once Erlang 18.0 gets released.

The guide has been reorganized and should be easier to follow.

doc/src/guide/handlers.ezdoc

@@ -0,0 +1,99 @@
+::: Handlers
+
+Handlers are Erlang modules that handle HTTP requests.
+
+:: Plain HTTP handlers
+
+The most basic handler in Cowboy implements the mandatory
+`init/2` callback, manipulates the request, optionally
+sends a response and then returns.
+
+This callback receives the ^"Req object^req and the options
+defined during the ^"router configuration^routing^.
+
+A handler that does nothing would look like this:
+
+``` erlang
+init(Req, Opts) ->
+    {ok, Req, Opts}.
+```
+
+Despite sending no reply, a `204 No Content` reply will be
+sent to the client, as Cowboy makes sure that a reply is
+sent for every request.
+
+We need to use the Req object for sending a reply.
+
+``` erlang
+init(Req, Opts) ->
+    Req2 = cowboy_req:reply(200, [
+        {<<"content-type">>, <<"text/plain">>}
+    ], <<"Hello World!">>, Req),
+    {ok, Req2, Opts}.
+```
+
+As you can see we return a 3-tuple. `ok` means that the
+handler ran successfully. The Req object is returned as
+it may have been modified as is the case here: replying
+returns a modified Req object that you need to return
+back to Cowboy for proper operations.
+
+The last value of the tuple is a state that will be used
+in every subsequent callbacks to this handler. Plain HTTP
+handlers only have one additional callback, the optional
+`terminate/3`.
+
+:: Other handlers
+
+The `init/2` callback can also be used to inform Cowboy
+that this is a different kind of handler and that Cowboy
+should switch to it. To do this you simply need to return
+the module name of the handler type you want to switch to.
+
+Cowboy comes with three handler types you can switch to:
+^"cowboy_rest^rest_handlers^, ^"cowboy_websocket^ws_handlers^
+and ^"cowboy_loop^loop_handlers^. In addition to those you
+can define your own handler types.
+
+Switching is simple. Instead of returning `ok`, you simply
+return the name of the handler type you want to use. The
+following snippet switches to a Websocket handler:
+
+``` erlang
+init(Req, Opts) ->
+	{cowboy_websocket, Req, Opts}.
+```
+
+You can also switch to your own custom handler type:
+
+``` erlang
+init(Req, Opts) ->
+	{my_handler_type, Req, Opts}.
+```
+
+How to implement a custom handler type is described in the
+^"Sub protocols^sub_protocols chapter.
+
+:: Cleaning up
+
+All handlers coming with Cowboy allow the use of the optional
+`terminate/3` callback.
+
+``` erlang
+terminate(_Reason, Req, State) ->
+    ok.
+```
+
+This callback is strictly reserved for any required cleanup.
+You cannot send a response from this function. There is no
+other return value.
+
+If you used the process dictionary, timers, monitors or may
+be receiving messages, then you can use this function to clean
+them up, as Cowboy might reuse the process for the next
+keep-alive request.
+
+Note that while this function may be called in a Websocket
+handler, it is generally not useful to do any clean up as
+the process terminates immediately after calling this callback
+when using Websocket.