Enhancements to Ginga FITS viewer

- Reorganized cairo drawing into its own module
  - will like refactor FitsImageGtk class to subclass FitsImageCairo
  - FitsImageCairo class allows rendering onto any cairo surface (PDFSurface,
     PSSurface, etc)
  - sample program in scripts/fits2pdf.py

- Added a colors module to make colors precise across toolkits
  - initially based on X11 rgb.txt defined colors

- Small enhancements to the Cuts plugin
  - typing 'v' while hovering the cursor will give you a vertical cut through
      the image
  - similarly, typing 'h' will generate a horizontal cut
  - removed "drawing" types of vertical/horizontal cuts until it has been
      thought through a little more