Timer now takes an optional "message" argument.

On __exit__, if "message" is not None, Timer will print its message and elapsed
time. This simplifies the Timer idiom:

    with systemtools.Timer() as timer:
        print('PitchHandler:')
        consort.PitchHandler._process_session(
            segment_session,
            )
    print('\ttotal:', timer.elapsed_time)

... becomes...

    with systemtools.Timer('\ttotal'):
        print('PitchHandler:')
        consort.PitchHandler._process_session(
            segment_session,
            )
Timer now takes both an exit_message and enter_message argument.
Added Meter.duration property.
Replaced "voice_name_abbreviations" with "context_name_abbreviations".
Extended Container.__contains__() to accept strings.

You can now test for the existence of a named child context like so:

    >>> 'TimeSignatureContext' in my_score
    False
Made TestManager.compare() more robust.
Adjusted one doctest.