implement functions instead of throwing unsupported

Instead of throwing unsupported, I've implemented empty data responses or something that vaguely correlates to the feature from node core. I've also made this module have 2 entry points: browser.js for browsers and main.js for node. This way you can `var os = require('os-browserify')` in your app and not need to worry about falling back to the core os version when you're in node.