Rework nova_limits to make use of new Turnstile features

Note that this breaks the unit tests; they will be updated
in a subsequent commit.
Forgot to reference tools for the @add_argument() decorators.
Use 'lims' in nova_postprocess() to avoid shadowing limits.
Rewrite tests for nova_limits.
Minor PEP8 issue in setup.py.
Update the README.rst.
Add support for setting the quota class to the rate limit class.
Update the README.rst to document the limit_class tool.
We don't use msgpack directly anymore.
We don't use argparse directly, either.
Forgot to update the convenience script for limit_class.
Minor docstring fix-up.
Make setup.py executable.
Bump to beta1 for release.