Add list of bitcoin.org contributors

Live preview: http://bitcointest2.us.to/en/about-us

This pull requests is adding a list of contributors to bitcoin.org in the "About bitcoin.org" page, both an hardcoded list and another one using GitHub API. This list is far from complete (I've only started asking people if they want credits here since a few months), but at least we would now have a dedicated place for this purpose.