make topojson work w/ browserify

I was trying to play with planetary.js today but found out that topojson doesn't work with browserify: https://github.com/BinaryMuse/planetary.js/issues/1

This PR wraps `topojson.js` in https://github.com/umdjs/umd/blob/master/returnExports.js#L40-L60, which gets rid of the `fs.readFileSync` in `index.js`, which gets rid of the need to use the `brfs` transform for use w/ browserify.

If you don't like this PR I can alternatively make a PR w/ https://github.com/maxogden/topojson/commit/bb1d7959f689a45ad129de061c7f012a58c4ad6d, which only modifies `package.json` and also makes this module work with browserify. 

In my opinion this PR is less complex overall than requiring `brfs` for browserification

Cheers!