Implemented BuildFileWrangler.generate_music_source().

Description: use 'music - generate lilypond source' (mg) automatially to create
a music.ly file in the build directory.

File contents will look like this:

    % Red Example Score (2013) for piano

    \version "2.19.4"
    \language "english"
    \include "/Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/stylesheets/stylesheet.ily"

    \context Score = "Red Example Score" {
       \include "/Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/build/segment-01.ly"
       \include "/Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/build/segment-02.ly"
       \include "/Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/build/segment-03.ly"
    }

Everything is generated dynamically:

    * title
    * forces tagline
    * LilyPond version
    * LilyPond language
    * stylesheet include
    * segment (ordered by current segment view)