Add option to export texture references to the active images of each face.  This is useful to some users.

Re-added the Coordinate System and Up Axis options.  Apparently XNA uses a right handed system and the option was useful for some users.  Oops!

- Corrected a couple comma formatting issues.

Added ExportSkinWeights to the condition for using an unrolled mesh enumerator.  For some very strange reason, it appears that DirectX will only transform a skinned vertex correctly for the first face that uses it.  All others will appear at the origin.  This can be worked around by using an unrolled mesh enumerator (in other words duplicating each vertex per face that uses it).  DirectX bug?

- Optimized vertex, normal, and skin weight export where possible.
- Largely decoupled data gathering and data writing.  This will make it much easier to provide binary file support in the future.
- Added animation option to export each action of each object as a separate AnimationSet and another option to export unused actions as if used by the first armature object.
- Added vertex color export support.