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
No `define_method` please. I know it's seductive to try to save a few lines, but I prefer to KISS in AM adapters. If there's duplicate code, either extract method or just duplicate.