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, also, please stick to the standard options as outlined in the [Contributing Guide](https://github.com/Shopify/active_merchant/wiki/contributing#options-hash).