Define the supported rcParams as code

Until now, the parameter definition was dispersed:
valid names and defaults are loaded from the
canonical `matplotlibrc` data file. Docs are only
available as comments in there. Validators are
attached ad-hoc in `rcsetup.py`.

This makes for a more formal definition of parameters,
including meta-information, like validators, docs in
a single place. It will simplify the rcParams related
code in `matplotlib.__init__.py` and `matplotlib.rcsetup.py`.
It also enables us to generate sphinx documentation
on the parameters.

Author: timhoffm