TST/BUG: do not call regular density tests when testing kde color
BUG: deal with gcf-style figures in hist_series
TST: add tests to deal with gcf-style figures

Also cleans up the check_plot_works function and tests for the valid return
types of a plotting function. These currently are Artist, dict, tuple, and an
ndarray of Axes.

Additionally it is best to close figures after plotting since unit tests should
avoid sharing global state
DOC: add release notes and whatsnew