Return a "NULL" time if no time is set.

This fixes an issue where an abstract base class with no tests could fail to
set a time object. See
https://github.com/test-class-moose/test-class-moose/issues/34
Doc tweak to explain duration if the test class was not run.
Add a 'passed' method to report objects
Doc tweak to show the passed() method.
v0.58

        * Add a passed() method class and method report objects (GitHub issue #30)
        * If a test class did not run (such as an abstract class), the time()
          method on reports now returns a zero duraction time instead of
          undef (GitHub issue #34)