Allow a jsonp response's callback name to correspond to the request

This resolves #55.

This pull request adds a `dynamic_jsonp` config flag that changes jsonp response callback names to match the associated request param. This combined with `ignore_params` lets a developer use the caching feature for jsonp requests with dynamic callbacks (where the callback name is generated based off a key or timestamp)

I also created a barebones sample rails app that uses stripe.js for integration testing. You can see it [here](https://github.com/MarkyMarkMcDonald/Sample-Striped-Puffer).