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/rest_handlers.ezdoc

@@ -1,20 +1,20 @@
 ::: REST handlers
 
-REST is implemented in Cowboy as a protocol upgrade. Once upgraded,
-the request is handled as a state machine with many optional callbacks
+REST is implemented in Cowboy as a sub protocol. The request
+is handled as a state machine with many optional callbacks
 describing the resource and modifying the machine's behavior.
 
-The REST handler is the recommended way to handle requests.
+The REST handler is the recommended way to handle HTTP requests.
 
 :: Initialization
 
 First, the `init/2` callback is called. This callback is common
 to all handlers. To use REST for the current request, this function
-must return a `rest` tuple.
+must return a `cowboy_rest` tuple.
 
 ``` erlang
 init(Req, Opts) ->
-    {rest, Req, Opts}.
+    {cowboy_rest, Req, Opts}.
 ```
 
 Cowboy will then switch to the REST protocol and start executing