Contact: use Data URI to download a vCard when it is not native app

Works fine on desktop browsers (not tested on old IE versions).

iOS: works fine on Safari with iOS 7, reported to not work before version 7; does not work with Google Chrome

Android: works fine with the latest version, does not work with Android 2.3 (normal browser just displays the content of the vCard, Google Chrome crashes)

Does it worth spending any time on having a better feature switch? (based on user agent I guess)