Change <asset> to <js-module> in plugin.xml

I recommend changing the <asset> tag to the <js-module> tag so that "SocialSharing.js" gets brought in automatically to the project. This makes it play nicer with PhoneGap/Cordova 3 as well.

There's no change here in actually using the SocialSharing plugin, which is awesome, and did exactly what I needed, by the way. Great job!