add support for pickling DXF entities

[mbway]

fixes #1231

DXFNamespace has custom __getattr__ and __setattr__ so the default pickling mechanism doesn't work. Instead, custom state management is implemented for DXFNamespace. The unit test attempts to pickle and unpickle every example dxf. On my machine this takes ~3 seconds.

Pickling may be useful to cache parse results or to send dxf documents to subprocesses in a multiprocessed application.