Add support for .svg raster images

Hi Stephan,
This is a clean re-write of my .svg raster image implementation against the re-architected LasaurApp.
The implementation adds a dependency on the Python Imaging Library (apt-get install python-imaging).

It still requires the G08 implementation to be added to the standard firmware. I've done an implementation for my Cortex-M4 based board, which may be useful as a reference. I would be happy to work with someone on the official firmware.

There is another branch "raster-dev" in my repository that adds a "pixels per inch" setting in addition to what is on this branch.
PPI has been very useful on my machine for vector cuts as well as setting the dot pitch for image rasters.
I suspect the jury would be out on its usefulness for the Lasersaur though.

Kind regards.
Richard.