Riemann shootout - small claw_package fix as well

This adds the "Riemann shootout" app to PyClaw, which includes some ways of experimenting with alternate Riemann installations.

The experimental CUDACLAW branch builds heavily off this, as it leverages the shootout code for integration and performance testing, so I'd like to land this PR to help with the CUDACLAW integration.

I should have broken off the pyclaw/src modifications into another PR.  If anybody has an issue with this, I'll split the PR.