Added an rc_context decorator

This avoid messing up rcParams for pyplot while preserving the
prettyplotlib colors.
Merge remote-tracking branch 'upstream/master'

Staying up to date with the latest improvements of the original
prettyplotlib repo.
Removed unused commented code.

This was replaced by prettyplotlib.colors.pretty decorator.
Added usual "pyplot environment setup" functions.

This makes it possible to keep the enhancements brought by the
rc_context manager. Indeed, the Axes object need to be created within
the right rc_context.

These functions should be used in replacement of the usual plt.subplots
and plt.subplots2grid functions to build the Axes later sent to ppl
plotting functions.
Merge pull request #25 from williampw/master

Added an rc_context decorator to preserve rcParams.