Added new StaffPosition class to pitchtools. NEW.

The class is an object oriented 'unit wrapper' similar to Duration, Multiplier
and Octave.

Example:

    >>> pitchtools.StaffPosition(-2)
    StaffPosition(number=-2)

The class currently implements only a read-only 'number' property.

The class will be integrated with ...

    * pitchtools.clef_and_staff_position_number_to_named_pitch()
    * pitchtools.named_pitch_and_clef_to_staff_position_number()
    * Clef.staff_position

... and so on.

NEW: should be mentioned in the 2.16 release notes.