[BioPython] Tweaking GenomeDiagram / Release of Genome Diagram 0.21

[Peter (BioPython List)]

There was a packing problem with GenomeDiagram 0.2 (missing new module 
Observer), which Leighton has fixed with the release of GenomeDiagram 
0.21, available here:

http://bioinf.scri.ac.uk/lp/programs.html#genomediagram

This also adds a dpi option to GDDiagram.write() for raster output - 
which is handy for generating high resolution PNG files.

Peter