Adding support for ujson

I added ujson support in a transparent way, ujson is defined as an extra-require.
To fix tests, I needed to modify tests evaluating strings, to evaluate Python objects. IMHO is the right thing to do for testing.
I also removed "test_will_prettyprint_json_in_debug_mode"'s test because ujson does not support the settings used for it. Anyway, JSON pretty-printing should be a client decision, usually it's made using a browser's extension. Testing debug mode features -not production ones- could prevent any kind of optimization like mine.