Added Respond.js support

* Added Git submodule [Respond.js](https://github.com/scottjehl/Respond) in `lib`
* Made necessary changes in Makefile
* Updated README to reflect Respond.js inclusion
* For development purpose had to change `git@github.com` to `https://` because it was giving `Permission Denied` issue as given [here](http://stackoverflow.com/questions/8197089/fatal-error-when-updating-submodule-using-git)