Add bower.json

This file allows this package to be installed using Bower. The package
has already been registered as `respond` in the Bower package registry.
It can now be installed by executing:

  bower install respond

More information on Bower can be found at:

  https://github.com/bower/bower
  http://bower.io
Merge branch 'bower' of https://github.com/lencioni/Respond into lencioni-bower
Small edits