Adds JSON body parsing to Request handling

[soundofjw]

Adds JSON support to request body (particularly useful for processing POST requests).

Directly tested via flask handler.

[rknLA]

This definitely solves a pain point, but I'm pretty keen on having this kind of stuff reviewed at this phase in the project.

There's a lot here that I need to think about in the longer term -- one of them is, how should Pale Contexts handle class methods.. The original idea behind the context is that it's a storage container, not a parser (and to be honest, my own build_args_from_request kind of violates this).

Also, there's no check here to validate that Content-Type is set to application/json before trying to decode it... What happens if someone sends application/x-www-form-urlencoded data in a request body?