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
@j-mutter well, to be fair, that document is referring to options passed to `purchase` and friends, not to `initialize`. We actually like having the options passed to `initialize` to match the gateway's credential naming as closely as possible.