Added a function to vert_mesh which calculates normals.

Vertex normals are calculated by first calculating the surface normals of all triangles
in the mesh. These surface normals are added to each constituent vertex of a given triangle.
After all surface norms have been calculated and the contributions added to all vertices
the vertex normals are normalised.

Added another vec3 to the vertex shader for the normals. Many more changes most be made to
the shader files to support lighting. See ogl tutorial 8.