added a global callback onComplete if the percent-keyframe parameters is...

... used (the latest one callback is being appended)

The issue:

````
el.animate({
    "0%":{opacity:0;},
    "50%":{opacity:1;},
    "100%":{opacity:0;}
}, 200, 'linear', function(){
    console.log('asd');
});
````

"Asd" is not being typed into console at all.

But:

````
el.animate({
    opacity: 1;
}, 200, 'linear', function(){
    console.log('asd');
});
````

is. This patch fixes that (first listing) approach. It simply translates ``calback`` parameter to ``"N%":{callback: callback}`` ignoring the param if the last keyframe has it's own callback.