Adds news about Spoon release 2.4
Merge pull request #99 from GerardPaligot/2.4-release
Merge pull request #93 from GerardPaligot/website
says that Spoon is OSS in the first sentence
Adds github ribbons on the top left in each page.
Adds a global variable to access at the release and snapshot versions.
Improves contributing section to specify the mailing list.