Implementing candidacy protocol. Extended IOManager.run_lilypond().

IOManager.run_lilypond() now features a candidacy=False keyword.

IOManager.run_lilypond() performs as before when candidacy=False.

IOManager.run_lilypond() generates a candidate PDF when candidacy=True.
IOManager.run_liypond() then compares the candidate PDF to any existing PDF. In
the case that the candidate and existing PDF compare equal, the existing PDF is
preserved (because there's no reason to save a newly generated file that
compares the same as an existing file). In the case that the candidate differs
from the existing PDF, then the existing PDF is replaced with the candidate
PDF.

Integrated candidacy into the segment 'output.ly - interpret' (oli) command.

Example:

    Red Example Score (2013) - segments - A

       1: __init__.py
       2: __make__.py
       3: __metadata__.py
       4: definition.py
       5: output.ly
       6: output.pdf
       7: versions/

    > oli
    Will interpret ...
      INPUT: /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/segments/segment_01/output.ly
     OUTPUT: /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/segments/segment_01/output.pdf
    Ok? y
    GNU LilyPond 2.19.8
    Processing `/Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/segments/segment_01/output.ly'
    Parsing...
    Interpreting music...
    Preprocessing graphical objects...
    Finding the ideal number of pages...
    Fitting music on 1 page...
    Drawing systems...
    Layout output to `/Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/segments/segment_01/output.candiate.ps'...
    Converting to `/Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/segments/segment_01/output.candiate.pdf'...
    Success: compilation successfully completed
    The PDFs ...
        /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/segments/segment_01/output.pdf
        /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/segments/segment_01/output.candiate.pdf
    ... compare the same.
    Preserved /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/segments/segment_01/output.pdf.

    >