Add SPECCTRA dsn export and session file (.ses) import.

Original patch by Josh Jordan and Dan McMahill.

Signed-off-by: Bert Timmerman <bert.timmerman@xs4all.nl>
Added documentation for the dsn exporter.
Added a copyright notice and license text.
Added encapsulation with quotes for netnames, components and pins/pads.

Unconditional encapsulation with quotes is now identical with dsn output of
FreePCB Ver 1.359 (translated by: FpcROUTE Ver 1.303R) and
CadSoft (EAGLE Version 5.10.0)

<todo>
Encapsulation could also be done conditional if the string contains a
dash "-" as this is done in Kicad's PCBNEW (20090216-final)
</todo>
Corrections on Ptr types.

Signed-off-by: Bert Timmerman <bert.timmerman@xs4all.nl>