Updated layout and structure

This commit updates the site layout, adding a navbar at the top
and update bootstrap to the latest version (using bootstrap CDN).

It also introduces a mechanism to automatically update the download
links using a jekyll "post" with the right front-matter. See existing
post _posts/2013-03-18-v2.5.0.md for an example.
Move the 'release' posts to a sub-directory of _posts
Add a python script to create release posts

The script looks in download/ to find the release files and generates
a markdown post with the correct front-matter for the given version.
Add 2.7.0.0 release

This commits adds the post and release files for bnfc 2.7.0.0