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
@ntalbott  Payhub doesn't support partial refund and that why It didn't accept amount parameter. 
Not sure what would be best for this case.