Update the exports line to work in node.js, browser and browserify

Currently revalidtor can't be ported to the browser through browserify, because it checks for the existence of window.
For context on this change see: https://github.com/substack/node-browserify/issues/293

Also makes the validate function available in window instead of window.json (inline with the documentation). See: https://github.com/flatiron/revalidator/issues/30