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 
Everything outside of the `check_request` block is already tested elsewhere, but it's probably good to keep 70-71 around in case the mocked data gets tampered with?