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
TST: add test for DataReader class
TST: add read_google test
TST: use builtin equality testing
CLN: use a temporary zipfile
BUG: make the pause nonzero but short
CLN: give everything that needs it a closing context manager
CLN: clean up the testing suite