Introduce touch icons for Chrome and Android, plus the optimal icon sizes and formats for our supported desktop browsers.

See also:
https://gist.github.com/bcherny/7743482
https://mathiasbynens.be/notes/touch-icons
https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/MobileHIG/IconMatrix.html#//apple_ref/doc/uid/TP40006556-CH27-SW1