Week 5

@mikedory  This updates it to use mongo for sessions and mongo for the data.

I toyed with using a process.env.MONGO_URL (heroku config variable) but decided to skip that for the time being.

This changes the canned data files (students.js and weeks.js) to use callbacks (in one commit), and then modifies them to use mongo (next commit).

If you want to check out the mongodb in action, you can hit it from that url that's hardcoded into server.js. I trust you not to destroy my database. :)