Load external locales for Node.js (fixes #46) and fix broken browser tests

Implemented dynamic locale plural function loading for Node.js. 

The browser tests were completely broken, apparently due to expect.js moving its default file location, so I fixed that as well to verify that there were no issues in the browser.

I wasn't quite sure about your testing standards, so I just added one test that tries to load "es" only in Node.js to verify that everything was working. If you'd prefer something different, let me know.