Allow to do require() of a module without the .js extension

Hey!, 

I've added a little line to allow doing requires without specifying the file extension (also updated the example so you can see exactly what I'm talking about)

**Side Note**: *I'm migrating from BrunchJS to GruntJS, because of low level customization (check out the Brunch approach of "hugging", you can take ideas from there ;) )*

Awesome work, by the way!

Cheers!,