Ignore output files

LilyPond outputs PDF and PostScript files. These should not be included
in the repository.
Improve Credo III

* Tighten horizontal neume spacing by using 16th notes instead of 1/3 quarter notes.
* Switch to letter sized paper.
* Simplify voice line code.
Create shared definitions file

Move styles and functions from Credo III into a file that can be included by other scores.
Credo I