Added bower.json

Hi there,

First of all thanks for your great lib! I would like to use it via bower, so I added a Bower.json. 
The only thing left to do is to run ```bower register Perimeter.js git@github.com:e-sites/perimeter.js.git```, in order to [register the package at bower.](http://bower.io/docs/creating-packages/#register)

Thanks a lot!