Use node-restify-validation for body validation

- Swapped node-restify-validation with my forked version (that supports body validation and custom error handling) in `package.json`
- Use `bodyValidation` in the game submission endpoint to handle validation of game data
- Implemented a custom validation error handler that sends back a response more consistent with our other error responses

Dependent on my fork of [node-restify-validation](https://github.com/aricha/node-restify-validation). The changes I've made in my fork to support body validation (and custom error handlers) can be viewed [here](https://github.com/aricha/node-restify-validation/compare/z0mt3c:master...master). Everyone will need to run `npm update` or `npm update node-restify-validation` locally in order for this to work.

Fixes #152.

r? @cvan @soedar @at-kevinlau