Added b64img liquid_tag
Updated b64img docstring
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