Bug/mirrored instances

Correct the exported vertex order (winding) for flipped instances. 
Which? The face.mesh(0) line? or the write_face() method?

I actually did not intend to include the face.mesh(0) code in this pull request since it is not related to mirroring. 

I made the change from face.mesh(7) to face.mesh(0) because all we need from the mesh is the points on the chance there was some performance improvement in requesting only points rather than requesting all data from the mesh. http://www.sketchup.com/intl/en/developer/docs/ourdoc/face?csw=1#mesh

I think I'll revert that line and submit a separate issue for it.