Add a bower.json file for the Bower package manager

I created this file in response to #74. It seems as if there is already a Bower package for nouislider (though I'm not sure if you created it), but Bower would probably want this file to exist anyway. I based the contents of this file off of your `nouislider.jquery.json` file and the result of the `bower init` command.

Notes:
- It seems Bower installs packages by grabbing the latest tagged release by default, so you will probably need to update nouislider's version if you want Bower to notice this new file.
- Please double check the list of main files and ignored files. I assumed that `nouislider.jquery.json` was not needed as a main file.
- I did not register this package with `bower register`, but it seems like somebody else already did.

References:
- [Bower website](http://bower.io/)
- [Registering Bower packages](https://github.com/bower/bower#registering-packages)
- [Defining Bower packages](https://github.com/bower/bower#defining-a-package)
- [Bower package index](http://sindresorhus.com/bower-components/)