Added 3D Polyline reading support.

Greetings devs,

I have successfully implemented reading of 3D Polyline, it was not that easy because it's a 'complex' CAD object (its presented in file not like a single structure, like Circle or LWPolyline), but now it works properly (including vertexes order).

Some optimizations were done, the main unsolved problem is connected with 'deleted' entities, but I am working on it.

Best regards,

sandyre