added ofMesh::drawFaceNormals & ofMesh::drawVertNormals

This draws vertex and face normals in an accessible way using ofLine. Probably not the fastest way to do it but very useful for debugging meshes.

Usage is like this:

```
ofMesh mesh = ofMesh::box(200, 200, 200);
ofSetColor(255);
mesh.draw();
ofSetColor(100, 255, 100);
mesh.drawVertNormals();
ofSetColor(255, 100, 100);
mesh.drawFaceNormals();
```

Drawing face normals has the same restriction as ofMesh::getUniqueFaces and only works with OF_PRIMITIVE_TRIANGLES.