static images in help do not appear when testing in planemo

When testing in Planemo, images included using the [method suggested in the Planemo docs](http://planemo.readthedocs.io/en/latest/standards/docs/best_practices/tool_xml.html?highlight=static#including-images) do not appear. They do appear to work, however, when the tool is installed from the toolshed in a galaxy instance.

In addition, the docs are missing the `$PATH_TO_IMAGES` variable that is used in other tools such as [slopBed](https://github.com/galaxyproject/tools-iuc/blob/master/tools/bedtools/slopBed.xml)