challenges: WIP beginning challenges feature. some basic CRUD & some

basic subscribe / unsubscribe functional (but buggy)

src/server.js

@@ -27,6 +27,7 @@ process.on("uncaughtException", function(error) {
 mongoose = require('mongoose');
 require('./models/user'); //load up the user schema - TODO is this necessary?
 require('./models/group');
+require('./models/challenge');
 mongoose.connect(nconf.get('NODE_DB_URI'), function(err) {
   if (err) throw err;
   console.info('Connected with Mongoose');