add track_data support to usaepay

Add magstripe support for the [USAePay Transaction](http://wiki.usaepay.com/developer/transactionapi#ccsale_-_credit_card_sale) gateway. 

Please review @melari @j-mutter 
cc @andrewpaliga 
I think I commented on this before, but if the method is `add_credit_card` it should be safe to assume that we're getting a [`CreditCard`](https://github.com/Shopify/active_merchant/blob/master/lib/active_merchant/billing/credit_card.rb) object passed in and not have to check if it has a `track_data` method.  We've had to do this before because other gateways used more generic [`add_payment`-y](https://github.com/Shopify/active_merchant/blob/master/lib/active_merchant/billing/gateways/moneris.rb#L152-L167) methods and it could be more than a CC.

It looks like this gateway only supports credit cards, so it's probably safe to omit this check here(and only need the presence check), but I could be wrong.