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 when you can convince all the gateways to settle on only two, sanely named, credential fields, I'm in :grin:

Until then, we've found at Spreedly it's hugely beneficial to have these mirror the real credential names since it's often hard to figure out without that what in the world "password" maps to when the gateway has 3 for difference purposes and actually calls what you need "secret key".

Back to this code, I'm now realizing these are used, but not as I'd expect; the `options` hash is getting cloned in each call to provide the base set of parameters, so these do have to be in the hash. @grepruby the preferred convention to use instead is to just rely on `super` to assign the `@options` hash, initialize the `post` hash as empty, and have an `add_credentials` private method that explicitly copies the credentials from the hash into the `post`.