add ability to read spreadsheets anonymously

oof, this code is really ugly now.
update docstring to GSpreadsheet with better info
refactor: don't store email/password

it's still there if you know how to get to it :), but it won't be my
fault.
refactor: regroup methods according to functionality
update readme with new auth code
add note about no python 3 support

I've only tested against 2.7 so that's in the classifiers. Older version
of python will probably work too.