Added center.x and center.y to sketch context

Hi Justin - 

I'm having a blast playing around with sketch.js!
I found that I often end up defining a 'resize' method simply to set center.x and center.y properties on the context so I thought I'd add it. I'm fairly new to pull requests, so please let me know if there's something I should do differently. Also note that I did not update the minified version with my change.

Thanks for all the inspiration : )
Cheers!