Feature: Add Payload Parsing to the gem

[bhox]

When users visit an App from BigCommerce, the request includes a signed payload. There is sample ruby code for it here: https://developer.bigcommerce.com/api/#processing-the-signed-payload

This should be added to the gem so it's super easy to parse the payloads. 👍

[bookernath]

Hi @benhoxie,

For the moment our philosophy is to keep the API clients rather low-level, assuming you already have some credentials and you just want to run requests against resources easily.

For people who want a strong example for how to handle the OAuth callback we maintain a sample app, here's the relevant code: https://github.com/bigcommerce/hello-world-app-ruby-sinatra/blob/master/hello.rb#L100