[ALOY-841] Add an `once` function for call-once global events

I use this function to avoid global events not releasing local objects:

Alloy.Globals.once = function(event, cb) {
    var _cb = function() { 
        cb.apply(this, arguments);
        // Clean up
        Ti.App.removeEventListener(event, _cb);
    };
    Ti.App.addEventListener(event, _cb);
};

Come handy when there is UI/async call interaction allowing to preserve a workflow (instead of continuation callbacks). Would be meaningful to add to Alloy eg Alloy.once("an:app:event", doOnceCb)?