added reward model assuming login works

Rebased off of #12 since it's required for this to function.

Usage:

```javascript
  var rewards = new RewardCollection('cat670022'); // rewardListing.json
  reward = rewards.get('prod123456'); // gets the partial reward detail that rewardListing gives.

  // To get the full one:
  reward.fetch(); // Grabs the full reward details

  // To get all the categories:
  var rewardCategories = new RewardCategoryModel();
  rewardCategories.fetch();
```