add track data support to litle

Adds track data + retail support to Litle / Vantiv. 
Based on p.177 of their [XML Reference Guide](https://github.com/LitleCo/litle-xml/blob/master/reference_guides/Litle_XML_Reference_Guide_XML8.25_V2.44.pdf?raw=true)

please review @odorcicd @melari
cc @davidseal 
Could you also check that the other fields are added (like that `<orderSource>retail</orderSource` etc. are in the request data)?