Implemented PitchRangeInventory.__illustrate__().

NEW: new functionality available in 2.15.

Consider the following inventory:

    >>> inventory = pitchtools.PitchRangeInventory([
    ...    '[C3, C6]',
    ...    '[C4, C6]',
    ...    ])

You can now say this:

    >>> show(inventory)

The resulting LilyPond file looks like this:

    >>> lilypond_file = inventory.__illustrate__()
    >>> f(lilypond_file)
    % 2014-03-19 08:38

    \version "2.19.3"
    \language "english"

    \header {
            tagline = \markup {}
    }

    \layout {}

    \paper {}

    \score {
            \new Score \with {
                    \override BarLine #'stencil = ##f
                    \override Glissando #'thickness = #2
                    \override SpanBar #'stencil = ##f
                    \override TimeSignature #'stencil = ##f
            } <<
                    \new PianoStaff <<
                            \context Staff = "treble" {
                                    \clef "treble"
                                    s1 * 1/4
                                    s1 * 1/4
                                    c'1 * 1/4 \glissando
                                    c'''1 * 1/4
                            }
                            \context Staff = "bass" {
                                    \clef "bass"
                                    c1 * 1/4 \glissando
                                    \change Staff = treble
                                    c'''1 * 1/4
                                    s1 * 1/4
                                    s1 * 1/4
                            }
                    >>
            >>
    }

Would be nice to be able to attach image files to commit messages.

See the PitchRangeInventory API entry for an example.