fix bug in table.py when specifying both column header text and color

row offset in table construction function was missed in case of specifying both text and color for column headers
table.py: added comments to previous change and fixed other bugs

1) Comment to clarify what we are offsetting and why.

2) Switched `rows' and `cols' in code to create header rows with
   color but no text, so our lists of empty strings have the
   right length.
table.py: clarified documentation
WIP: added some basic image tests for tables with headers. tests need to be tested
fixed table test script and images
Make it possible to add mpl.rcParams to itself or deepcopy

Before it was not possible to set all rcParams to mpl.rcParams or use
 copy.deepcopy on mpl.rcParams.

There were two problems:
 1. validate_bool_maybe_none(None) was raising ValueError
 -> fixed by accepting this value
 2. svg.embed_char_paths was deprecated by svg.fonttype but both have
 different valid values (bool vs strings).
 -> fixed by adding a "value translation" lambda to deprecatedmap
 which translates the boolean values to the string values of
 svg.fonttype.

Also changed all validators, which accept None to check strings
without case ("none" vs "None").

Test for this: mpl.tests.test_rcparams.test_Bug_2543()

Closes: #2543
fixes issue #2556

Fixes it in two ways:

1) changes the closure of `on_dpi` to be over a weakref, not `self`
2) cleans up the call back when the quiver is removed
pep8 fix
Create interpolation_methods.py
Rename examples/api/interpolation_methods.py to examples/images_contours_and_fields/interpolation_methods.py
Update interpolation_methods.py
fix bug in table.py when specifying both column header text and color

row offset in table construction function was missed in case of specifying both text and color for column headers
table.py: added comments to previous change and fixed other bugs

1) Comment to clarify what we are offsetting and why.

2) Switched `rows' and `cols' in code to create header rows with
   color but no text, so our lists of empty strings have the
   right length.
table.py: clarified documentation
WIP: added some basic image tests for tables with headers. tests need to be tested
fixed table test script and images
Merge branch 'patch-1' of github.com:bjcohen/matplotlib into patch-1
Fix dpi detection in tight_bbox (fixes #2586).
backend_pgf: use the bounding box as clip rectangle (fixes #2586)
Don't snap pcolor