onLayout callback introduction

I found myself in a situation where I needed a callback triggered every time masonry resizes the container holding all items (so the scrollbar replacement plugin can be updated with a new container height), and was surprised that there is none.

So here is a pull request with `onLayout` callback :) Fixes an issue this guy had: #42, and kinda touches this issue: #159

[Demonstration](http://lab.darsa.in/masonry/onlayout.html)

Don't know why do you think that after animation callbacks are "out-of-scope for Masonry", because it is a pretty useful thing to have in a plugin like this, but please accept this pull request. I hate using customized plugins - updating is hell :)