add python logging module support

[mhosken]

pysilfont has a sophisticated logging mechanism, but it doesn't make use of the python logging module. Many modules do support such logging (for example fonttools). It would be good if pysilfont core were to add python logging module output into pysilfonts log at an appropriate level to the logging level requested to pysilfont.

I might even go further and suggest that pysilfont's logging be restructured around the python logging module.