Pull all routes into a separate router

[mmmaaatttttt]

Problem description:

Related to #46 but should probably happen first. The app uses a weird pattern to attach the routes to the express app. See https://github.com/rithmschool/numbers_api/blob/master/app.js#L124 and https://github.com/rithmschool/numbers_api/blob/master/routes/api.js#L99.

Suggested solution:

Refactor to use express.Router. This will also be beneficial so that we can silo all of these routes and create a separate router for a v2 of the API.