Add OAuth 2.0 support

[famzah]

This commit adds OAuth 2.0 support, which currently is the only method for a script to authenticate to the Google platform. The "sample_util.py" is used (imported) by the following samples:

• blogger/BloggerExample.py
• sites/sites_example.py
• docs/docs_v3_example.py
• analytics/account_feed_demo.py
• analytics/mgmt_feed_demo.py
• analytics/data_feed_demo.py
• apps/marketplace_sample -- there is a copy of "sample_util.py" in the "gdata/" subfolder

I've verified the change by testing with "samples/sites/sites_example.py".

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[famzah]

I signed the CLA.

[googlebot]

CLAs look good, thanks!

[famzah]

Anyone? :)

[gildotdev]

+1

[aliafshar]

Thanks.