Updated logging to make clearer messages direction

I prefer something like (from -> to) to better understand the message
exchange.
Updated to use a d3.js example

jQuery has been removed from now since it's not really needed for the
example.

d3.js has been downloaded to make the example work offline.

Data is generated randomly in the javascript side, but the idea is to
generate the data in the python side and send it to the browser widget.
Updated html page to use just d3.js
Improved documentation

Also removed some unneded code in main loop
Added clarification regarding loop

I tried to use a priority queue to handle gtk/webkit messages and also
to enqueue a 'quit' message, so that the signal handler uses the same
queue. However, when the main thread is waiting for the queue, the
signal handler is not called when pressing Ctrl-C until a message in the
queue is received.