Add support for PayHub Payment Gateway. Support transation purchase, voi...

@j-mutter  @shopify-admins @shopify-deployer @shopify-hooks 
Have added support for Pay Hub Payment gateway. http://www.payhub.com/
Currently it only supports purchase, void and refund transactions. Please take a look and let me know if I missed anything. 

Thanks
Yeah I'm on the fence with that one. The whole point of a library like ActiveMerchant is to provide a single, universal interface. This isn't the place for this discussion, and it would take some work to clean up the existing gateways, but we should try to be as consistent as possible when adding new ones. Also, they're not used.