Tangents

Added tangents to ```TriMesh``` and ```VBO``` classes. This allows the use of normal maps, bump maps and height maps on your model. A ```recalculateTangents()``` method is available to create the tangents from the existing normals and texture coordinates. The VBO binds the tangents to multi-texture coordinate slot 7, due to the lack of a pre-defined OpenGL keyword for tangents.

If desired, I could create a test application to test-drive it. For now, know that I have successfully used it to normal-map a model loaded from an OBJ-file, which has no support for tangents - which is where ```recalculateTangents()``` comes in.