using polylines for DXF export

Everytime I send DXF files generate by openscad to our laser cutting service provider, they complain that the vectors are "all broken apart". I ended up figuring out that it meas the current DXF exporter generates several independent line segments. 

So this patch uses DXF polilynes, instead, with makes it much nicer to do further editing of the generated DXF vectors.

happy hacking,
Felipe Sanches
metamaquina.com.br