#3 Add support for G36 and G37 codes (polygon fills)

[AJessop]

To support G36 and G37 gerber gcodes, a list of vertices which define the outline of a polygon, should produce a solid fill on the photoplotter film. Add code to parse and plot the polygon. The implementation should provide support for complex polygons. A suggested polygon filling algorithm can be found at http://alienryderflex.com/polygon_fill/.

[AJessop]

Initial work has started. My initial cut needs to be cleaned up and tested a little better.

[AJessop]

Added in revision 5.