Drop the googleapiclient copy, and cleanup tests.

PTAL @skelterjohn -- this is the first of several cleanups, but is mostly mechanical and (hopefully) a quick review.

FYI @orestica

This drops the duplication of `googleapiclient` inside the `oauth2client` repo,
which involved several steps:

* Delete the actual `googleapiclient` files.
* Remove the duplicated tests, since they're also hosted in the
  `googleapiclient` repo.
* Add a copy of three testing-related functions back in, since they're also
  used in the tests here. This was `HttpMock` and `HttpMockSequence` in
  `tests/http_mock.py`, and `assertUrisEqual` in `tests/test_oauth2client.py`.

Once that was done, I did just the tiniest bit of cleanup in the tests:

* Remove the now-superfluous `_oauth2client_` in the filenames for all the
  tests.
* Drop unneeded files from `data/`.
* Add `py26` back as a `tox` testing environment.
* Temporarily disable the `appengine` test for both environments.

In a coming PR, I'll actually clean up the GAE test to work correctly in the
presence or absence of `dev_appserver`, but this isn't a bad first step. (In
particular, my next PR will just add a `.travis.yml`.)