Key-Value dictionnary validator.
This validator allows validating serialized (or deserialized) JSON values. An valid JSON value is a serializable/deserializable key-value dictionnary.
Usage: Let validator = DictValidator(), then
* validator.validate('{"foo":0.5}') -> {"foo":0.5}
* validator.validate('{"foo":0.5,"bar":[1,2,3]}') -> {"foo":0.5,"bar":[1,2,3]}
* validator.validate('foo') -> ValueError
* validator.validate('True) -> ValueError
* validator.validate(''false') -> ValueError
* validator.validate(''5.5') -> ValueError
* validator.validate('[5.5,3]') -> ValueError
* validator.validate([5.5,3]) -> ValueError