FatZebra gateway fixes & improvements

I've fixed a bug in the Fat Zebra gateway, and added a couple of helper methods to make the API more consistent with the rest of Active Merchant.

* Added the ability to pass a card token directly to `#purchase`, e.g. `gateway.purchase(100, "card-token")` instead of `gateway.purchase(100, {:token => "card-token"})`
* Added `#token` method to the response object to provide easy and consistent access after calling `#store` to tokenize a credit card
* Fixed a bug where an exception was thrown trying to use nil as a hash, if the response was unsuccessful