Add .extend capability to Route

- Introduce dependency on class-extend for class .extend method
- Add Route.extend
- Convert UserRoute in route test to use .extend
- Fix broken `npm test` by adding missing express dev dependency
- Update README with shorter extend syntax example

Hey, I saw your project on the [Node Weekly](http://nodeweekly.com/issues/37) newsletter and thought this might be a good contribution to make it even closer to the `Ember.Route.extend` practice.  Found a pretty neat module called [class-extend](https://github.com/SBoudrias/class-extend) that worked pretty well for it.

If you like it, you can probably also use it for the other lib classes you want people to extend.  I kept it limited here unless it made you want to throw up in your mouth a little when you saw it.