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 
It doesn't match the recommended test data:

![mercury development testing information 2014 1 pdf 2014-10-04 12-51-32 2014-10-04 12-51-33](https://cloud.githubusercontent.com/assets/2962326/4516163/c35863cc-4be6-11e4-9c12-f49a50d9cd2c.png)