[README][xs]: add license.
[layout][xs]: add link to source code in footer.
[fixes #12,extpage][s]: github activity now shown on extensions page.
[search,ux][s]: make it clear to users that search has had an effect by "flashing" number of extensions matched.
[fixes #9,search][s]: search form now supports simple search.

* Also play around to fade in / out results to make fact that search has happened clearer to user.