Fixed the "notebook in version control" script in CONTRIBUTING.

Previously, since the notebooks were built up as a dictionary in the
filter, the output changed between platforms.

The script now includes a "sort_keys=True" in json.dump, which ensures
cross-platform stability of notebooks.
Mention testing in CONTRIBUTING guide + spellcheck.

Fixed a couple of spelling mistakes, and mention testing in CONTRIBUTING
guide.
Improved instructions on managing notebooks with git.

The instructions used to just point to a Stack Overflow question. I have
now added them explicitly to the CONTRIBUTING guide.
Merge pull request #38 from pbugnion/contributing_fix

Update contributing guide.