Provide instructions for testing a ValueConverter using json string as input/output

[jasperblues]

This would allow starting with the json snippet that will be reused, and writing a unit test for the converter before plugging it all in.

[alexgarbarev]

ValueConverters are decoupled with JSON format, because TRC supports not only JSON. So, you can test your TRCValueTransformer with NSNumber or NSString input (in case of JSON)

[jasperblues]

:+1: I think it would still be useful to unit test this common scenario easily, before integrating with whole system. Perhaps a base class to extend or something.