Added __illustrate__() to Component and LilyPondFile.

The method returns the object wrapped in a LilyPondFile.

Removed Component.lilypond_file property.
Added systemtools.Timer context manager:

    >>> timer = systemtools.Timer()
    >>> with timer:
    ...     for _ in xrange(1000000):
    ...         x = 1 + 1
    ...
    >>> timer.elapsed_time
    0.092742919921875