VMD importer: Lamp motion import, interpolation data import, and some bugfixes.

f5453b0 is same as a pull request #4.

4b14399 adds light source motion import to vmd importer.  A Blender lamp will be converted to a mmd_lamp (empty + lamp + armature).  The armature is used for converting Cartesian coordinates of light direction to rotation angles.

7582785 and 8f69cc7 add interpolation data import to vmd importer.  The importer simply sets bezier curves for each component.  That's not exact because not doing a spherical interpolation, but I believe we should be able to obtain much nicer results.

4948a91 corrects some typos in warning messages that will be displayed for bones/shapekeys not found.

79b19c1 fixes a bug that an incorrect roration mode "XYZ" is used for camera (empty object).  The correct one is "YXZ".