Modularize everything and correct drawing order

Now everything (including scale bar) is in a module.
Since the transparency was not drawn correctly, we fiddled around with
the order and put all the drawing code to the end of the file to make
this fiddling easier.
Ignore OS X files
Changes for Progress Report

- Cosmetic changes to Ommatidiag.scad
- lens_simulation.py now accepts the CMOS-Distance as optional input.
  This can be used to draw a movie. Figures are drawn bigger, a bit less
  colorful and with consistent axes.
Merge branch 'master' of github.com:habi/GlobalDiagnostiX