Add bower.json

This file allows this package to be installed using Bower. I have
registered the package as `jquery-popupwindow` in the Bower package
registry. It can now be installed by executing:

  bower install jquery-popupwindow

More information on Bower can be found at:

  https://github.com/bower/bower
  http://bower.io
Merge pull request #4 from lencioni/bower

Add bower.json