Add prettyprinting for JSON

This commit adds prettyprinting to JSON output:

1. It creates a `settings` dict in the JSON representation module that contains kwargs to be passed to `json.dumps`
2. When the app is in debug mode (`app.debug` is True), then the representation will set reasonable defaults for prettyprinting JSON - specifically, indenting by 4 spaces and sorting by keys.  Note that both of these values will not be set if there is an existing setting for them.

Tests are also added.

Ref: #70

I didn't add documentation, since I wasn't really sure where to put it.  I'd be happy to add it if you have a preferred location.