Merge pull request #166 from selenaaa/mult_nbs

Fixes delay in buttons appearing for both diff and merge
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()
added some more tests
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.
Remove duplicate tests
Merge pull request #168 from LinaNouh/difftests

added some more tests
Flip merge arrow buttons to undo a merge choice.

Squashed commit of the following:

commit 495c4b5a49b2f42813ee52f88bace0e22c437560
Merge: 5483909 efec3d0
Author: Tavish Armstrong <tavisharmstrong@gmail.com>
Date:   Tue Mar 4 20:37:20 2014 -0500

    Fix merge conflict.

commit 5483909ea5512b2718b10f55946fbcd34d37f911
Author: bsplay <bsplay@github.com>
Date:   Sat Mar 1 14:09:31 2014 -0500

    making jshint happy

commit 31f58e711dce98055df59732d61ae45470671df7
Author: bsplay <bsplay@github.com>
Date:   Sat Mar 1 13:55:39 2014 -0500

    drag/drop changes arrow

commit af8f67db97285cd6f72aa09e86a3fa8bbffdf6bd
Author: bsplay <bsplay@gmail.com>
Date:   Fri Feb 28 13:06:46 2014 -0500

    undo arrow
s/tabs/spaces/ in drag_drop.js
Use swc notebooks as test diff notebooks
Add actual diff of IPythonReviewTime.ipynb to example-notebooks folder
Make diff lines monospace/whitespace-significant
Merge pull request #171 from tarmstrong/diff-line-style-fixes

Diff line style fixes & extra notebooks for diff testing script
Add note on current stability :-)
Add --check argument for profiling/testing purposes.
Run nbdiff and nbmerge commands on travis.