Updates from review with Greg

[toppa]

• Remove the yield block from the signature checker. Everything it's returning is data that's already available within the scope of the controller, so it's not adding value.
• In the Profile model initializer, use a loop to assign all the instance variables from the config data
• Allow for subclassing the Payment model. This provides a flexible way for gem users to add optional fields (like shipping address) and do things like decide if card_cvn should be required or not, without us having to get heavily involved with form configuration
• Improve the signature checker, so it's clearer what kind of signature checker you're asking for.
• Change the CybersourceResponseHandler to be something simpler - a ReasonCodeChecker. There was too much bundled into the ResponseHandler. This approach gives the gem user more flexibility in how they handle responses from Cybersource, and makes redirecting to a "success url" entirely optional.