A few style tweaks

I'm proposing here a few style tweaks to our developers site.

- Code blocks are now syntax highlighted (note: can't use Pygments as it requires a plugin, not compatible with GH Pages, see [here](https://github.com/mvdbos/kramdown-with-pygments) and [here](http://jekyllrb.com/docs/plugins/) for more info; so am using [HighlightJS](https://highlightjs.org/), which is easy to set up and works fine I think)
- Changed link color so they pop out more; also changed titles so they are not confused with links
- Changed the font to Open Sans, which I find more readable than the previous one?
- Centered the site with a max-width of 800px (everything was a bit off to the left before)