Making asyncRequire simpler to use (non backward compatible).

This commit changes the API of asyncRequire.

Old usage (no longer valid):

asyncRequire(["a","b"]).then(function() {
   var a = asyncRequire("a");
   var b = asyncRequire("b");
   // use a and b
});

New usage:

asyncRequire("a","b").then(function(a,b) {
   // use a and b
});