Automatically stub build in node modules when they are required. Fixes #2

As requested in my issue. 

I added fibers as a npm dependency because fibers will be a commonly used node module across all meteor applications.

This will not work for modules that are not packaged with node. We should think of a system where we can add extra modules to the meteor-stubs via some kind of setting. Although most people probaply use [the npm package](http://atmospherejs.com/package/npm) for other npm modules.