Add docstrings to a few functions.
Refactor merge.py, notebook_diff.py, diff.py; add docstrings.
Refactor notebook_diff.py tests into a separate file.
Add test for diff_modified_items()
Fix import of diff() in notebook_diff.py
Merge pull request #167 from tarmstrong/refactor-python

Refactor notebook-specific diff logic out of diff.py into notebook_diff.py
Fix merge conflict.