test: add an example async test suite.
matchers: rename get{Failure,Success}MessageFor => get{Failure,Success}Message.
ActualValue: rename a private field to better reflect reality.
matchers: implement getSuccessMessage, allowing negations to report failures.
matchers: rename tdd.Matchers.* => tdd.matchers.*.
all: rename Expectation => Expectations and ActualValue => Expectation.