Add description metadata to posts

Close #4 - Merge page.meta_description and page.description so it's more natural for posts to get a meta description
Update "Optimize selectors, CSS vs JS" post

Moderate the warning about universal selector in CSS as it's not a concern if correctly used since 2012
FR - jQuery, data et performance
Fix local linenos issue

Fix #6 - My local version of the jekyll gem is outdated and doesn't want to go further than v0.12.1 yet
I took the highlight.rb plugin from the jekyll project v1.0.0.beta2 and use it as a personal one for parsing
I added few style for the eventual linenos in pygment.less
EN - jQuery, data and performance
Typo correction in "jQuery, data et performance"
Trailing whitespaces and useless linenos removal