Made the CairoPainter object a private variable to the class again instead of a local var in WriteMolecule(). I got errors when trying to depict multiple molecules otherwise.
Made hash bonds draw with line width 1, required adding the function GetPenWidth to the different painters to be able to reset the pen width after drawing the bond.
Added the -b and -B options to pngformat to allow changes of colors when creating pngs.
Added -xm option for possibility to "crop" output image to a close bounding box of a single molecule.
Changed the metrics calculation in cairopainter to give nicer layout of text labels with sub and superscripts.
Merge pull request #19 from fredrikw/viz

Png depiction changes