Trimesh uint32 t

related to [issue 345](https://github.com/cinder/Cinder/issues/345).  index type was only size_t in TriMesh2d, TriMesh is actually already uint32_t.  So this makes it consistent, with the bonus of working on 64-bit platforms.

Verified that all samples using Triangulator / TriMesh2d now work on Mac OS X 64-bit.