Add Sphinx extension for glossary referencing: `ipypublish.sphinx.gls`

• Added Sphinx extension for glossary referencing: ipypublish.sphinx.gls.
  See :ref:sphinx_ext_gls

• Added ConvertBibGloss post-processor,
  to convert a bibglossary to the required format

• Added notebook-level metadata options for bibglossary and sphinx
  (see :ref:meta_doclevel_schema)

• Large refactoring and improvements for test suite, particularly for testing
  of Sphinx extensions (using the Sphinx pytest fixtures) and creation of the
  IpyTestApp fixture

• fixes #71

Back-compatibility breaking changes:

• renamed Sphinx notebook extension from
  ipypublish.ipysphinx to ipypublish.sphinx.notebook
  (see :ref:sphinx_ext_notebook)

• ipypublish.postprocessors.base.IPyPostProcessor.run_postprocess
  input signature changed (and consequently it has changes for all post-processors)

v0.9:

   def run_postprocess(self, stream, filepath, resources):
      output_folder = filepath.parent

v0.10:

   def run_postprocess(self, stream, mimetype, filepath, resources):
      output_folder = filepath.parent