Add Code Samples for Multi-Image Requests

[todd-elvers]

We've gotten a few reports that it is not very clear how to make a request for multiple images at once. We should add a few extra code samples illustrating how to make these requests.

Making a multi-image request

• Use the application/json accept header (causes JSON to be returned instead of binary)
• Pass in a value greater than 1 for the samples argument

Alternative

We could switch the code samples to the application/json format and simply make mention of the alternative accept header value of image/png. That seems like a more common use-case and won't throw an error if they bump samples above 1.

[todd-elvers]

Switched all examples to application/json. This has been deployed to production.