Added Playfire rewards for qualifying games.

I have extended your wonderful extension to look up playfire rewards and display them on qualifying games. This way when browsing steam you could tell what games had prize, how to get the prize and what the prize was.

There is currently in issue with playfire. If you are signed into playfire and using this plugin at the same time playfire only returns incomplete rewards. I do have a support ticket in. This doesn't really cause an issue or anything you just wont see any rewards you have already gotten.

#### What it looks like if all rewards are visible
![Middle-earth™: Shadow of Mordor™](https://cloud.githubusercontent.com/assets/564874/4475876/d72fe000-4970-11e4-998c-87d41fcbd09a.PNG)
#### What it looks like when you complete rewards
![shadow of mordor rewards - mouse0270](https://cloud.githubusercontent.com/assets/564874/4475905/0efbe024-4971-11e4-9195-a23fbfb99b80.PNG)
#### What it should look like once playfire fixes their bug (Already coded no update will be needed)
![shadow of mordor rewards - what it should look like](https://cloud.githubusercontent.com/assets/564874/4475974/929a7b3e-4971-11e4-8808-9f7af7094344.PNG)