Describing how to create credentials explicitly in auth doc.

[dhermes]

Also moving the from_service_account_* factory descriptions into the newly added sections.

@jgeewax The only thing I plan on adding after this is

• Short snippet about how to get a JSON / P12 keyfile
• Short tip about scopes in GCE
• "Advanced Customization" section about using a custom implementation for auth (see Enable / document BYO HTTP Objects #908 (comment) as an example)

---

As before (towards #805 and #830)

To view my and your original PRs (as we go), here are some hosted files:
www.googledrive.com/host/0B8el7dRo8mVOUVlkSEJKN3pQdkk (dhermes)
www.googledrive.com/host/0B8el7dRo8mVOOHE4NzFXSmhHUWs (jgeewax)

[dhermes]

@jgeewax I just realized that the Google OAuth docs link to a super old document:
https://developers.google.com/api-client-library/python/guide/aaa_oauth#flow_from_clientsecrets (this is the Google doc)
http://google-api-python-client.googlecode.com/hg/docs/epy/oauth2client.client-module.html#flow_from_clientsecrets (this is the super old link)
http://oauth2client.readthedocs.org/en/latest/source/oauth2client.client.html#oauth2client.client.flow_from_clientsecrets (This is the newest / shiniest link)

Who to report to?

[dhermes]

@jgeewax What remains here? AFAIK I've been able to address your concerns.

[jgeewax]

👍 LGTM !