Document solver constants and indices.

The comments in each normal Riemann solver now include:
- The number of waves
- The number of conserved quantities
- The number of auxiliary variables
- The names and indices of the conserved quantities and variables

A short script extracts these values and writes them to Python modules
so they can be imported and used in Python scripts.

Ideally, this would be implemented without the extra Python modules
by putting the Riemann solvers in modules and using module data.
This is a simple solution that doesn't require any drastic changes.
Make travis run nosetests from pyclaw/examples.
Merge pull request #77 from ketch/constants_and_indices

Document solver constants and indices, making values available in Python scripts