Expand example-oauth.py 

Expand example-oauth.py to include all the steps that are required in 
a real client application.  Add a lot of comments.  This was surprisingly
painful to work out from all the bad documentation at Smugmug and oauth.
Hopefully it will be clearer now to others.

Add fetch_image method on Smugmug to GET image data.

Minor improvements to error messages and doc on the Smugmug class.