Expand the example-oauth.py to include all the oauth steps that are required
in a real client application.  Add a lot of comments.

Add fetch_image method on Smugmug to GET image data.

Minor improvements to error messages and doc on the Smugmug class.
Add support for fetching private images by stuffing oauth credentials
in the header
Factor out some of the _url_fetch code into more stages to make it
easier to share with the fetch_image method