Added testing for DXF export
Added support for Geometry::Polyline in DXF export

For now Polylines are exported to a series of LINE primitives. Some day this should probably be changed to use the LWPOLYLINE primitive.
Added support for Geometry::Square in DXF export

For now Squares are exported to a series of LINE primitives. Some day this should probably be changed to use the LWPOLYLINE primitive.
Added support for Geometry::Arc in DXF export
Exporting Polygons to SketchUp should use add_face instead of add_curve

add_curve is used by Polyline because it doesn't automatically close the curve
Allow using the Sketch builder DSL to build Polygon and Polyline instances
Export extrusions to SketchUp as transformed Points instead of as definitions

Now SketchUp's pushpull feature can be used for simplified CSG operations