Replace all refs

Took a first crack at implementing the 'all' keyword and added some tests .

Tried to be a minimal as possible; the tests I think cover the most common case -- wanting to monkey patch some function (here, example is json.dumps) and making sure *all* references to that object are covered. 

Also, I made a decision to now allow the 'all' parameter to be used with simple scalars like strings, ints, True, False -- think it might be more trouble than it's worth -- but maybe also worth talking about. 

Andrew