Merged experimental code in 'stable' addon (mainly Jens Restemeier's summer work).

Will remove experimental one soon. Keeping old 6.1 exporter active for now, though.

Notes:
* Not 100% happy with how complex code is now... Rather sure though refactor would
  take a huge amount of time, not worth it for now.
* New code handles things like armature & animation import much much better than previous one,
  so definitively worth it.
* New code also fixes some real bugs (like Blender-side of T42135).
* Did a few cosmetic changes on the run, nothing serious.

This should make FBX work complete, aside from a few nice TODOs that I'll tackle in comming weeks,
in addition to ongoing bugbusting.

Many thanks to Jens for his work on armature mess! :D