ENH Add colormap= keyword to DataFrame plots

Refactor color and colormap option handling

Add tests for colormap=

Add tests for colormap=

Add colormap documentation to visualization.rst

Add release notes
Merge pull request #3860 from qwhelan/plotting_colormap

Add colormap= argument to DataFrame plotting methods
CLN: Allow evaluate to take eval_kwargs + testing

Now evaluate takes eval_kwargs (generally just truediv) to
numexpr.evaluate plus adds a set of testing methods to check
that numexpr was actually used successfully.

Also changes the if hasattr idiom --> getattr.
TST: Make test utilities more expressive.

Adds a better error for ``isinstance`` checks
as well as a better error message for the ``isnull`` case.
TST: Test all arithmetic ops + frame default_axis

Makes the entire arithmetic test suite explicit as well as sets up test
cases to make sure the default_axis responses do not change.
ENH: Add bind_method to core/common
ENH: Add classmethods to generate arithmetic operators in core/generic
ENH: Use numexpr + normalize signature throughout.

BUG: Fix ``_fill_zeros`` call to work even if TypeError
(previously was inconsistent).
ENH: Abstract arithmetic into core/generic

SparsePanel has to opt-out because it doesn't respond to ``shape``.

ENH: Add flex comparison methods to Series and Panel
CLN: PEP8 the tests/test_expressions
DOC: Document entire range of arithmetic methods in api.rst
TST: Flesh out test cases for Series and Panel to include all arithmetic ops