allow FP style callbacks by applying _object

this way, its easier for functional programmers to say
tween…..onComplete(Level.destroy).start()

While Level.destroy accepts an object as method, while Level is just a namespace for functions.

Usually in FP, the data is applied as last param for more flexibility, see Ramda.js vs the wrong way in _underscore

https://www.youtube.com/watch?v=m3svKOdZijA

Hope this wont break something else but so far i havent gotten any problems.

Cheers!