Added b64img liquid_tag
Updated b64img docstring
Implemented gallery also for static pages
Little improvement on tests

The relative notebook import in `test_notebook.py` has been changed to
work in python 2&3. A note has been added to explain how to process
these tests.
Merge pull request #308 from mclate/master

Liquid tag for inline images (base64 data uri)
Merge pull request #332 from benjaminabel/improve-liquid-tags-tests

Little improvement on tests
Added python3 support to img and b64img tags

This is done using a new six import, however as `six` is used by pelican
itself, this do not add a requirement.
Merge pull request #334 from benjaminabel/python3-support

Add Python 3 support to img and b64img for liquid_tags plugin
Merge pull request #325 from bugnano/master

Implement gallery also for static pages
Closes issue #312
Markdown 2.5 changed the config system for markdown extensions. Inspired by
the render_math plugin, I added code to cope with both versions. New settings
used in the markdown part must go in LT_CONFIG and LT_HELP in `mdx_liquid_tags.py`.