Add Clawpack + Git Submodule test

Clawpack uses relative submodules, so it's a good test of our submodule
functionality.  This does not test the parallel aspects of Clawpack, but
this could be added by installing a PETSc installation.
Merge pull request #185 from hashdist/add_clawpack_test

Add Clawpack + Git Submodule test