[WIP] Signature Calculator

@bslobodin /cc @Shopify/payments First draft of the signature calculator. Still needs some testing and an option to make some of the request data optional before we actually put this online.

![screenshot 2014-07-29 13 30 46](https://cloud.githubusercontent.com/assets/633549/3738940/4fb34466-1746-11e4-80a8-cb6ea3b34274.png)