ISO C solver example and solver_compare.py

This PR is a second attempt to get Aron's riemann_shootout branch merged.  This brings in two things:

- An example showing how to set up an ISO C solver.  This is now in apps/iso_c_advection/
- A script showing how one might run some easy comparisons of different solvers on different problems.  This is in apps/compare_solvers.py.

@ahmadia, hopefully I didn't mess anything up.
@mandli, what do you think about merging this now?