Enhanced Container.extend().

Container.extend() now strips out Selection objects.

Example:

    >>> selection_1 = scoretools.make_notes([0, 2], [Duration(1, 4)])
    >>> selection_2 = scoretools.make_notes([4, 5], [Duration(1, 4)])
    >>> selection_3 = scoretools.make_notes([7, 9], [Duration(1, 4)])
    >>> selection_4 = scoretools.make_notes([11, 12], [Duration(1, 4)])
    >>> selections = [selection_1, selection_2, selection_3, selection_4]
    >>> container = Container()
    >>> container.extend(selections)

    >>> f(container)
    {
        c'4
        d'4
        e'4
        f'4
        g'4
        a'4
        b'4
        c''4
    }

    >>> inspect_(container).is_well_formed()
    True

Doesn't need to be noted in the 2.15 release notes.