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.

liquid_tags/Readme.md

@@ -152,4 +152,13 @@ comment line ``# <!-- collapse=False -->`` will be open on load but
 can be collapsed by clicking on their header. Cells without collapse
 comments are rendered as standard code input cells.
 
+### Run unitests
+
+The file `test_notebook.py` contains tests that can be run using [nose](https://nose.readthedocs.org/en/latest/index.html)
+
+```
+cd path/to/liquid_tags
+nosetests
+```
+
 [<a name="1">1</a>] http://ipython.org/

liquid_tags/test_notebook.py

@@ -2,7 +2,7 @@ import re
 
 from pelican.tests.support import unittest
 
-import notebook
+from . import notebook
 
 
 class TestNotebookTagRegex(unittest.TestCase):