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
Agreed here as well. Metaprogramming for the sake of metaprogramming leads to more complex code that is harder to parse, search, etc.  If these methods are almost identical, put the shared logic into a private method that they can both call.