Make the relative URL normalization actually work

- Now relative links within a model can load, also work with relative
  imports of embedded workspaces.
More test cases

- For integrating the loader into the CellML API utility, with various
  permission settings for the different types of objects.
Add in the mercurial live tests.

- Also had to prune defaultBases down to a single element to avoid
  causing plone.app.testing and/or plone.testing to die in a fire.
  Rather extremely annoying that it isn't working as expected or
  advertised, even.