Python-ly

- fix transposing when alterations would be more than a double sharp or double
flat; handle it by moving the note, just like LilyPond does it
- Python3 installation fixes
- small MusicXML export improvements, contributed by Peter Bjuhr (MusicXML
export is still very experimental)

- new command 'highlight' ('hl') to create syntax-highlighted HTML files of
LilyPond source files (or any file that is understood by ly.lex)
- new, very experimental, command 'musicxml' to export music to MusicXML

- new commands 'abs2rel' and 'rel2abs' that convert \relative music to
absolute and vice versa
- support for Python 3. Not all of the ly python module has already been
tested, but installing and running the various ly commands works well.

- initial release