Refactor /users route

Use Express 4 Router API. See #11
Update test specs
Validate Document.update calls

Hack for #12 - Since `update` calls skip validation on current
Mongoose (3.8.x) versions, I'm triggering it manually here pre-update.

This is utter shit and I should be banned from GitHub for even submitting this, but fuck me, it works.

I'll be back here whenever Mongoose 4 is around so I can refactor this fucker.
Merge pull request #13 from js-uy/gf-express-router

Use Express 4 Router API