add 2D Navier-Stokes SUPG example

This example demonstrates Streamline Upwind/Petrov-Galerkin stabilization with flow through a 2D channel. The stabilising effect can be seen upstream of the geometric constriction in the SUPG vs. non-SUPG results. (Default run type is SUPG, run with "-SUPG false" for classic GFEM.)
Merge pull request #60 from dladd/SupgExample

2D Navier-Stokes SUPG example