Improved package.json

Added some fields to package.json for publishing on npm. Also, the 'main' field was required so that the package could work with Browserify.

Since Box9 doesn't reply to the issues, I've ended up publishing 'jss' myself on npm under the name of 'jss-browserify'. I would gladly hand the ownership of the npm package to Box9 once he comes back. I didn't use the name 'jss' since it's already used by a different package.

I've updated the README with Browserify instructions.