Add highlight.js library to client-side highlighting of code.

OK, this PR propose the integration of the highlight.js library.
With this integration the highlighting of the code is client-side performed.
highlight.js provides an heuristic detection of the code suitable to be colored, so this implementation makes the highlighting **intelligent** and **automatic**.

I also add all the available themes and other things with the idea to make this more extensible, for example, selecting the theme of coloring and so on... ;-) But first I need to get some time... he he!

I hope you enjoy it!

Damián.

PS: Sorry to post it later, a lot of things to do ;-)