Refactor WebSockets and add support of optional callback functions

Main changes:
  * Fix some bugs about UTF-8 encoding and messages fragmentation
  * Add support of optional callback functions
  * Add support of many startup options
  * Add support of outgoing fragmented messages
  * Add a websocket testsuite

                                 - * -
*** bug fixes ***

First of all, an huge part of yaws_websocket.erl was rewritten to fix bugs
about the messages fragmentation and the UTF-8 encoding of incoming text
messages:

  * UTF-8 encoding
    before, when a text message was fragmented, only the first frame was
    checked and partial UTF-8 sequences were not supported. Now, checks
    are done on each message part and a partial UTF-8 sequence at the end
    of a frame is accumulated and checked with the next frame (for basic
    callback only).

  * Messages fragmentation
    for basic callback modules, because of a buggy mapping between frames
    and messages, the messages fragmentation was almost unusable. To fix
    this, the message handling was rewritten.

Now, all tests in the autobahn testsuite[1] pass successfully.

                                 - * -
*** Optional callback functions ***

Then, from an idea of François de Metz[2], yaws_websocket module was
extended to support optional callback functions. See the documentation for
details (www/websockets.yaws).

Quickly, optional callback functions are:

  * Module:init/1           (for basic and advanced callback modules)
  * Module:terminate/2      (for basic and advanced callback modules)
  * Module:handle_open/2    (for basic and advanced callback modules)
  * Module:handle_info/2    (for basic and advanced callback modules)
  * Module:handle_message/2 (for basic callback modules only, used in place
                             of Module:handle_message/1)

Thanks to Pablo Vieytes[3] which added handle_info to optional callback
functions.
                                 - * -
*** Startup options ***

To start a websocket process a script must return the following term from
its out/1 function:

  {websocket, CallbackMod, Options}

where 'Options' is a (possibly empty) proplist. Following parameters are
supported:

  * {origin, Orig}
  * {callback, Type}
  * {keepalive, Boolean}
  * {keepalive_timeout, Tout}
  * {keepalive_grace_period, Time}
  * {drop_on_timeout, Boolean}
  * {close_timeout, Tout}
  * {close_if_unmasked, Boolean}
  * {max_frame_size, Int}
  * {max_message_size, Int}
  * {auto_fragment_message, Boolean}
  * {auto_fragment_threshold, Int}

See the documentation for details (www/websockets.yaws).

                                 - * -
*** Outgoing fragmented messages ***

A callback module can now send fragmented messages to clients using the
record #ws_frame{}:

 #ws_frame{fin     = true,  %% true | false
           rsv     = 0,
           opcode,          %% text | binary | continuation...
           payload = <<>>}. %% binary(), unmasked data

--
[1] http://autobahn.ws/testsuite
[2] https://github.com/klacke/yaws/pull/99
[3] https://github.com/pvieytes
Fix typo in documentation