Merge pull request #16 from stanislasrybak/master

installation.mako layout.
Merge pull request #17 from stanislasrybak/master

Installation and page-avec-nav layout.
Add news and Atom feed.
Remove redundant body_attributes.
Improve wording.
Add support for DatedFormula.
Merge pull request #18 from stanislasrybak/master

"Attention" Layout.
Add support for SelectFormula.
Merge branch 'master' of github.com:stanislasrybak/openfisca-web-site into stanislasrybak-master

Conflicts:
	openfisca_web_site/templates/tree/documentation.mako
	openfisca_web_site/templates/tree/variable.mako
Add external link for partenaires.