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 @j-mutter 
Have changed as per @j-mutter suggested. 
Also :tid and :orgid is mendatory field for api. http://developer.payhub.com/api#api-fieldref 

Thanks!!