Adds tests for plot_boject.add_xml_filters

Adds two tests for the add_xml_filters method of a plot object, a round
trip of a non-tree-modifying filter, and a simple tree modifcation that
adds a sub-plot of change bars to a bar plot.
Merge pull request #126 from sirlark/xml_filters

Adds tests for plot_boject.add_xml_filters