Added Accelerando to indicatortools.

Accelerandi format as LilyPond markup.

Accelerandi are not followed by any type of dashed line or other spanner.

Use accelerandi with a tempo spanner to generate dashed lines and other
spanners.

Example:

    >>> staff = Staff("c'4 d' e' f'")
    >>> accelerando = indicatortools.Accelerando()
    >>> markup = Markup('foo')
    >>> attach(accelerando, staff[0])
    >>> print(format(staff))
    \new Staff {
        c'4 ^ \markup {
            \large
                {
                    \italic
                        {
                            accel.
                        }
                }
            }
        d'4
        e'4
        f'4
    }