Refactor mechanism for saving files.
Move all print_ methods to their backends.
Manage default format-to-backend mappings in backend_bases.
print_figure() can now determine the backend for saving figures.
Remove print_ code duplication, add canonical FigureCanvas names.
Better documentation for print_figure refactoring.
Merge pull request #2588 from pwuertz/print_refactor

Refactor mechanism for saving files.
colors: Replace docstring ''' with """
colors: Add power-law normalization
power_norm_demo: Initial commit
hist2d: Add a note mentioning PowerNorm
test_colors: Fix typo