Oauth2client-master

* Big thanks to dhermes for spotting and fixing a mess in our test setup.

* Fix a serious issue with tests not being run. (86, 87, 89)
* Start credentials cleanup for single 2LO/3LO call. (83, 84)
* Clean up stack traces when re-raising in some places. (79)
* Clean up doc building. (81, 82)
* Fixed minimum version for `six` dependency. (75)

* Several small bugfixes related to `six`/py3 support.

* Fix a critical bug on import in `oauth2client.tools`.

* Merge python3 branch! Massive thanks due to pferate and methane for doing
the heavy lifting.

* Make `oauth2client.tools` import gracefully if `argparse` isn't present.

* Change `flow.step2_exchange` to preserve the raw `id_token` in the
`token_response` field.

* Quick bugfix for an issue with dict-like arguments to `flow.step2_exchange`,
which is common in some environments (such as GAE).

* Quick bugfix for bad error handling in from_json.