Update pygments.rb to 0.5.1

The previous relying on the github option was here to fix an issue with
Python versions. Since this patch has been applied to 0.5.1, let's
update our bundle!
Let's upgrade Redcarpet to 0.3.0
Rely on a simpler theme for the blog

The previous theme was not yet finished and a bit too much for the
site. This one is very light and should be easier to maintain.
Minor CSS fixes for code blocks and the header

Fix some margins and also remove borders on pre code elements.
Move index.html to index.markdown and improve it

Improve the homepage with external links, further explanations and
move the old HTML file to a plain Markdown file (except for the blog
posts which are always wrapped into HTML elements such as li).