Added ApplicationFee class

Not sure if this is suitable for inclusion - application fee objects don't really seem to be fully launched yet. The API isn't publicly documented anywhere, which suggests it is probably subject to change, and changes to an `ApplicationFee` will **not** trigger a `charge.updated` event.

However refunds against application fees **do** show up in the logs, and they are pretty important for anyone making an application using Stripe Connect.

I have added in support for listing, retrieving and refunding application fees, which matches the functionality in the dashboard. Given the only documented way of creating them is at the same time as a charge, the semantics of creating and updating wouldn't be clear anyway (if possible).

**SIDENOTE**

As application fee ids aren't included in the charge object, the next best way to find them is to use

```ruby
Stripe::ApplicationFee.all(:charge => some_charge_id)
```