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 you still want the `.present?` check here, otherwise a `nil` or empty string value will likely cause problems. In the same vein, having tests for these false-y situations would be nice :)