Convert all highlight tags to markdown fenced code blocks

Hello,

duplicate of https://github.com/propelorm/propelorm.github.com/pull/204 but with a fix in the configuration file to force Jekyll to use redcarpet. I think it should works but I can't test locally  because of an issue with Pygments.rb and Python 3.3 and I can't downgrade my Python version.

Can someone test locally ? Like that :

```
$ git clone https://github.com/robin850/propelorm.github.com -b markdown-tags
$ cd propelorm.github.com
$ gem install jekyll redcarpet
$ jekyll --server
```

Then go to http://localhost:4000/

Thanks, have a nice day.