Subscribe to bundle load event.
@param {string[]} bundleIds - Bundle ids
@param {Function} callbackFn - The callback function
function subscribe(bundleIds, callbackFn) {
	      // listify
	      bundleIds = bundleIds.push ? bundleIds : [bundleIds];
	      var depsNotFound = [],
	          i = bundleIds.length,
	          numWaiting = i,
	          fn,
	          bundleId,
	          r,
	          q; // define callback function

	      fn = function fn(bundleId, pathsNotFound) {
	        if (pathsNotFound.length) depsNotFound.push(bundleId);
	        numWaiting--;
	        if (!numWaiting) callbackFn(depsNotFound);
	      }; // register callback


	      while (i--) {
	        bundleId = bundleIds[i]; // execute callback if in result cache

	        r = bundleResultCache[bundleId];

	        if (r) {
	          fn(bundleId, r);
	          continue;
	        } // add to callback queue


	        q = bundleCallbackQueue[bundleId] = bundleCallbackQueue[bundleId] || [];
	        q.push(fn);
	      }
	    }