Use relative path in the Main layout.

In case I re-use the code for posts that are generated by jekill in
subdirectories.
Split layouts in a home and main one.

Move commond code in _includes.
Improve relative urls trick.
Add settings page.
Keep the footer at the bottom of the page.
Add post and all_post layouts, style and configuration.
Add global variables with github and extension page urls.
Rework homepage: shorter description and news.
Add news section with all posts.
Add initial news and release posts.