add track_data support for Mercury

Add support for track data to [Mercury Payments](http://developer.mercurypay.com/docs/core-data-elements-xml-request/) gateway.

@jnormore @j-mutter can you please review?

cc @louiskearns 
Yeah, I believe these are the updated credentials, they told me to use them.

![mercury development testing information 2014 1 pdf 2014-10-04 12-52-31 2014-10-04 12-52-32](https://cloud.githubusercontent.com/assets/2962326/4516169/fde47d3c-4be6-11e4-8a23-588840821b8a.png)