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Add ability to iterate over the authenticated user's teams

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Closes #141. Closes #143

Finish adding the Contents CrUD API

We were trying to send the list of Label objects to requests and json.dumps
was throwing a TypeError since the repr of a Label is <Label [labelname]>
instead of labelname. So we need to "normalize" the labels before sending them
to the JSON serializer.

Thanks to @alejandrogomez for the bug report. This is a huge issue.

Oh yeah, the GitHub object can now be used as a context manager! Woohoo! Stole
the inspiration from requests.Session.

It should be pushed out in the next few moments.

And fix RepoComment.update

Bring code coverage back up to 100%. Fix the failing API tests. Update the
docs. Update doc-strings with links to GitHub's API docs in the repos module
only though.

Also, add a fun example to that effect.

* Add documentation about GitHubIterator with an example
* Add documentation about the new logging introduced, with an example for
  setting it up
* Allow GitHubIterators to be properly refreshed.
* Add etag parameter to all of the API iter_* methods