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 (see: section 2.7 of OpenGL's Vertex Specification). An alternative would be to use a vertex attribute instead.