New code highlighter

Per [#12](https://github.com/less/less-docs/issues/12#issuecomment-53914198).

This also includes a new highlighting stylesheet but it's quite weird and I consider it as a temporary. Actually I'm not a big fan of too coloured syntax highlighting but it was just too tempting to demonstrate the possibilities (though not every detected Less/CSS entity is actually coloured). So any ideas on what should, what shouldn't and how to be coloured, please...