Added top-level detach() function.

Removed InspectionAgent.detach() method.

Example score:

    >>> staff = Staff("c'8 d'8 e'8 f'8")
    >>> time_signature = marktools.TimeSignatureMark((2, 4))

Attach syntax:

    >>> attach(time_signature, staff)

Detach syntax:

    >>> detach(time_signature, staff)

Note that the following also works:

    >>> detach(marktools.TimeSignatureMark)

The next commits will remove all calls to old-style detach().