Generate download zip files

@pfiller @stof 

This resolves #1313 and #1314.

The purpose of this PR is to remove generated files from the repo while making it easy to download the latest version (and master) files from the index page of http://harvesthq.github.io/chosen/.

I've created two tasks:

* Create a zip file of all the assets
* Update the latest version download link in index.html

Now when you run `grunt release`, it will also zip up the files and update the index.html file accordingly.

There is one issue that needs to be addressed. You need to remember to run `grunt zip` before you push to master to make sure it's alway up to date. I think we can mitigate this by updating `watch` to just generate the non-minified files, and update `build` to generate all, including the zipped files. Thoughts?