Add support for SelectFormula.
Add news and Atom feed.
Remove redundant body_attributes.
Improve wording.
Add support for DatedFormula.
Add external link and layout changes.
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