Reduced many prior changes by moving out previously added code into googoauth.py instead.  Simplified function calls. Dicts rewritten as namespace objects.
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	modified:   gspread/client.py
Added an exception for catching OAuth2 mishaps
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	modified:   gspread/exceptions.py
Clean up development cruft
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	modified:   gspread/httpsession.py
Drastic cleanup.  Absorb many tasks that were added, unsuitably, to client.py. Wrap dicts as namespaces where possible.
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	modified:   gspread/googoauth.py
Don't let real test example parameters appear in public
Refactored to simplify and clarify. Moved SMTP stuff to external utility.
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	new file:   prepSMTP.py
	modified:   test_gOAuth.py
	modified:   test_parms.py.example