Added support for empty distribution boxplots.

Support added at lower level distribution_plots.py code. Need to add tests and then work on make_distance_boxplots.py code to make sure that it will support empty distributions as well.
Updated existing distribution_plots.py tests.
Added tests for empty distributions and coloring.
Updated pairwise tests text to be clearer.
Merge pull request #740 from jrrideout/issue_716

Added support for plotting empty distributions in box plots.