Support for Balance Transactions

I switched to a recent API version in Stripe and the fee is no longer a simple part of the Charge object.  You have to get the balance transaction for the charge and look up the balance transaction object.  (https://stripe.com/docs/upgrades#2013-08-13)

I can't find a .net API library that supports this.  So I modified yours.

I disabled charge.fee, but perhaps it should be left in to support both the old API and the new API.