Implemented BuildFileWrangler.generate_back_cover_latex().

The (bcg) command is now available.

Example 1: when no back cover LaTeX file exists:

    Blue Example Score (2013) - build files

          files - copy (cp)
          files - new (new)
          files - remove (rm)
          files - rename (ren)

          segment lys - copy (lycp)
          segment pdfs - copy (pdfcp)

    > bcg

    Blue Example Score (2013) - build files

       1: back-cover.tex

          files - copy (cp)
          files - new (new)
          files - remove (rm)
          files - rename (ren)

          segment lys - copy (lycp)
          segment pdfs - copy (pdfcp)

    >

Example 2: when a back cover LaTeX file already exists:

    Red Example Score (2013) - build files

       1: back-cover.pdf
       2: back-cover.tex
       3: draft.pdf
       4: draft.tex
       5: front-cover.pdf
       6: front-cover.tex
       7: music.ly
       8: music.pdf
       9: preface.pdf
      10: preface.tex
      11: score.pdf
      12: score.tex
      13: segment-01.ly
      14: segment-02.ly
      15: segment-03.ly

          files - copy (cp)
          files - new (new)
          files - remove (rm)
          files - rename (ren)

          segment lys - copy (lycp)
          segment pdfs - copy (pdfcp)

    > bcg

    Overwrite /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/build/back-cover.tex? y
    Overwrote /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/build/back-cover.tex.

    >

DATA MANAGEMENT: the command respects the value of any 'paper_dimensions'
metadatum created in score setup. That value is parsed and inserted into the
generated LaTeX.