Exposed methods for waiting for views based on accessibilityIdentifier or *any* predicate.

Predicates provide a clean way to test specific details of an object
while still providing a clear error message on failure.  Accessibility
identifier is a trival case of this, but I use it multiple places for
the `instancesRespondToSelector:` test, and lots of people want it.
[tester tapStatusBar]

Introduced a method to tap the status bar.  Resolves #306.