MOBILE-485 Basic implementation of isPluginVisible function

isPluginVisible is an optional function to be implemented in a plugin,
if is undefined we assume that the plugin is visible
the lib/mm.cordova.js is a code duplication fix not related with that
issue at all, but it doesn’t worth a bug
Merge pull request #36 from moodlehq/MOBILE-485

MOBILE-485 Basic implementation of isPluginVisible function