Support AMD loaders.

If "define" is available, it uses the function to declare a dependency on
jQuery and then returns exports.OAuth when finished. If "define" is not
available it proceeds as previously designed.

With this setup, we can use RequireJS to bring oauth.js in to our
application just like any other AMD library.

    // This is some app.js
    define(['lib/oauth'], function(OAuth) {
        OAuth.initialize('...');
    });