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
FWIW I just clean up indentation when I merge; I've given up asking people to make it consistent in PR's :confounded: