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
Could you please incorporate the [`STANDARD_ERROR_CODE`s](https://github.com/Shopify/active_merchant/blob/401b9f571221d57fc88846bcdaee4bd49b34e910/lib/active_merchant/billing/gateway.rb#L80-L93) here?