Added option to overwrite existing stored credentials.

Added an option to django_orm.Storage locked_put method to allow for
overwriting an existing credential record rather than the creation of a
new one.
Add note about contributing page to README, and convert to markdown.
One small markdown tweak in README.md.
Add a simple .gitignore.
Minor markdown tweaks.
Drop the googleapiclient copy, and cleanup tests.

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`.)
Merge pull request #5 from craigcitro/drop_apiclient

Drop the googleapiclient copy, and cleanup tests.
First travis.yml for oauth2client.

This adds a simple `.travis.yml` with support for python 2.6, 2.7, and pypy,
and compacts a few lines in `tox.ini`.
Add build status shield.
Wrote doc string for new storage parameter.

Added proper doctoring for the overwrite parameter in Django ORM
storage and updated an unused variable.
Merge branch 'master' of git://github.com/google/oauth2client
Merge pull request #2 from lraccomando/master

Added option to overwrite existing stored credentials.
Merge branch 'master' of https://github.com/google/oauth2client into merge