Refactor to use stripped(?) Google API Client libraries

[amv]

This would make the code easier to read and maintain.

Currently there is however a possible problem with the library size, because the PIP installed version of the python libraries weigh 4.5Mb, which might affect the cold start time on the app engine too much:

$ pip install -t libtest google-api-python-client
$ du -sh libtest
4.5M	libtest

Before this can be started, we need some benchmarks on how adding 4.5Mb of code payload affects the cold start time on GAE. Also any information on how & how much the Google API client libraries can be stripped would be of help to process this ticket.

Currently I don't know of reliable ways to test the GAE cold start time, and even if there is more than one type of the cold start time scenarios in GAE.

For reference, the current vendor directory weighs 240 Kb while containing helpers for handling oauth2, http and xml processing, which allow interacting with the REST API quite easily.

[amv]

Before the attempt, one should also check that the API client can speak Sheets API v3, because at least for now, API v4 does not support listfeeds.