Added bower.json to make available via Bower Package Manager

[Bower](http://bower.io/) has become fairly popular for managing front-end dependencies, so it'd be nice to be able to install it this way. I added the bower.json file and all you'd need to do afterward (once you have bower installed) is run the following:

```
bower register pure-sass git://github.com/fourseven/pure-sass.git
```