Make the raw api client public accessible on Client

Currently, it's impossible to access the raw `Call` interface to the Stripe API from the library.  While we generally want to guide people towards using the resource-specific interfaces, I think it's useful to expose the raw API for advanced users.  In talking with Andrey Petrov about our Python libraries, one of his main complaints was that we don't provide a way for advanced users to more directly access the API.  I have a need for this in the new checkout-api in order to return the raw JSON from an API request back to the client.

As an alternative, we could simply expose the implementation as public separate from the `Client` struct (e.g. rename `s` to `ApiClient` or something).

r? @cosn