v0.7.5 (fixes)

- fixed Custom (Angle-Based) auto-generation mode. It's slightly slower
(as in, it can take about 5 min for a 3600 poly mesh), but should no
longer produce weird results on complex meshes.
- added check for UV layer before calculating tangents. If not found,
the mesh is exported without tangents (default behavior)
- fixed wrong vertex being selected while using manual edit (bad math)