fix to support breves and longas in musicxml2ly

[David Fletcher]

 When I ran musicxml2ly on a music XML file containing breves e.g.
 
         <duration>16</duration>
         <voice>1</voice>
         <type>breve</type>
 
 it crashed in ly_expression with 
 ValueError: negative shift count
 
I fixed this by changing musicexp.py in the python folder, replacing line
  152:         str = '%d%s' % (1 << self.duration_log, '.'*self.dots)
with
  152:         if self.duration_log >= 0:
  153:             str = '%d%s' % (1 << self.duration_log, '.'*self.dots)
  154:         elif self.duration_log ==-1:
  155:             str = '\\breve%s' % ( '.'*self.dots)
  156:         elif self.duration_log ==-2:
  157:             str = '\\longa%s' % ( '.'*self.dots)
  158:         else:
  159:             str = '\\maxima%s' % ( '.'*self.dots)   #error: durations >
longa not supported

Sorry if that's not the format for a fix.
http://lilypond.org/web/devel/participating/ says
"If you want to contribute fixes of any kind, use the following procedure 
Check out the latest sources via GIT at git.sv.gnu.org 
    git clone git://git.sv.gnu.org/lilypond.git"
I don't use Unix/Linux and the only git I know is me (an old one - a systems
programmer since 1964).  I have never written any Python before but the above
fix seems to work, at least for breves.

David