Add StreamSummary to generate summary statistics and capture failures.

This is equivalent the to behaviour in the TestResult class, but split into a
dedicated helper (so that decorators and the like don't need to waste cycles
generating them).
Add a dedicated interface for controlling an in-progress TestRun.
Add an adapter to convert ExtendedTestResult to StreamResult.

This permits using code that uses any Python unittest test code
with a StreamResult.
Add an adapter to adapt StreamResult event streams into ExtendedTestResult.

This permits running existing tools like test runners and reporters with
StreamResult generating test runs adapting any unittest or testtools compatible
TestResult into a StreamResult.