Cleaned up systemtools.RedirectedStreams.
Added new system tool: ProgressIndicator.

Use as a context manager to help monitor the progress of some procedure. Each
time the `advance()` method is called, the progress indicator will overwrite
the current terminal line, incrementing the progress count.

Basic usage:

    message = 'doing something'
    with systemtools.ProgressIndicator(message) as progress_indicator:
        for _ in range(1000):
            do_something()
            progress_indicator.advance()