use --standalone instead of --igv in browserify step to enable proper AMD usage

I just ran into the same issue as @chiplay in #4 

I fixed this by using browserify's `--standalone` flag instead of exporting via `--igv` (see this article for a straight forward explanation: http://www.forbeslindesay.co.uk/post/46324645400/standalone-browserify-builds).

This also eliminates the need for a dedicated `for-browserify.js` file.

In the long term I think it might be worth discussing omitting the hard dependencies from an AMD build as most people (me at least) might want to satisfy them themselves (via bower).