Added RehearsalMark to indicatortools.

Example:

    >>> staff = Staff("c'4 d' e' f'")
    >>> score = Score([staff])
    >>> mark = indicatortools.RehearsalMark(number=2)
    >>> attach(mark, staff[0])
    >>> scheme = schemetools.Scheme('format-mark-box-alphabet')
    >>> set_(score).markFormatter = scheme

    >>> f(score)
    \new Score \with {
            markFormatter = #format-mark-box-alphabet
    } <<
            \new Staff {
                    \mark #2
                    c'4
                    d'4
                    e'4
                    f'4
            }
    >>

Works the way you expect.
Merged master.