Add grunt build script and zip download link

Git thinks that I moved `backbone.courier.js` to `dist/Backbone.Courier.js` so it says that I wrote all of `src/Backbone.Courier.js` even though all I did was remove the license (which is now stored in the `Gruntfile.js`).  

The grunt config is the same as how it is in backbone.collectionView, just run `grunt` and it builds the production and developmet version into `dist` and creates the zip.

I added a download link for the zip in the `README.md` as well.