Added BeatDivisionMaker to makertools. NEW FUNCTIONALITY.

Example: beat-groups (ie, depth 1):

    >>> maker = makertools.BeatDivisionMaker(
    ...     depths=[1],
    ...     )
    >>> lists = maker([(7, 4), (6, 4)])
    >>> for list_ in lists:
    ...     list_
    [Division(3, 4), Division(2, 4), Division(2, 4)]
    [Division(3, 4), Division(3, 4)]

Example: beats (ie, depth 2):

    >>> maker = makertools.BeatDivisionMaker(
    ...     depths=[2],
    ...     )
    >>> lists = maker([(7, 4), (6, 4)])
    >>> for list_ in lists:
    ...     list_
    [Division(1, 4), Division(1, 4), Division(1, 4), Division(1, 4), Division(1, 4), Division(1, 4), Division(1, 4)]
    [Division(1, 4), Division(1, 4), Division(1, 4), Division(1, 4), Division(1, 4), Division(1, 4)]

Example: alternating time signature and beat groups (ie, depth 0 and 1):

    >>> maker = makertools.BeatDivisionMaker(
    ...     depths=[0, 1],
    ...     )
    >>> lists = maker([(7, 4), (6, 4), (5, 4), (4, 4)])
    >>> for list_ in lists:
    ...     list_
    [Division(7, 4)]
    [Division(3, 4), Division(3, 4)]
    [Division(5, 4)]
    [Division(1, 4), Division(1, 4), Division(1, 4), Division(1, 4)]