Adding schemas for representing challenges.

Adding some missing "var"s in db.js.
Adjusting challenge route to take a mongo object id.
SQUASH ME: Adding old course view as challenge.jade and renaming associated js file.
Redesign of the challenge route and view.

- The route now uses the challenges ObjectId from mongo in the url
- The directory tree structure in the view is generated from the
  information in the database
Squash me maybe. Adding simple workerResponse middleware lib.
Squash me too. Updating challenge route for testing.
SQUASH ME: just so we can see the stuff.
Using evironment variables in config file for use on heroku.
Removing zmq dependency for heroku testing.
SUCCESS: enqueing workers and recieving a response over rabbitmq now works.