Add bower.json

This file allows this package to be installed using Bower. I have
can now be installed by executing:
registered the package as `get-device-pixel-ratio` in the Bower package registry. It
  bower install get-device-pixel-ratio

More information on Bower can be found at:

  https://github.com/bower/bower
  http://bower.io
Merge pull request #1 from lencioni/bower

Add bower.json