Making docs tox target fail on untracked changes.

[dhermes]

Checking if sphinx-apidoc generates new content and failing the script if the content is not checked in to version control.

Fixes #257.

@nathanielmanistaatgoogle I debated adding a tox target that would just generate the new content, but tox -e docs will still do this before failing, after which a user could then

$ git add docs/
$ tox -e docs  # This time it won't fail

[dhermes]

@nathanielmanistaatgoogle If you want to see it in action you can check out https://github.com/dhermes/oauth2client/tree/fake-change and run tox -e docs.

If you already have the code checked out locally (from google/oauth2client)

$ git remote add dhermes https://github.com/dhermes/oauth2client/
$ git fetch dhermes
$ git branch fake-change dhermes/fake-change
$ git checkout fake-change
$ tox -e docs

[nathanielmanistaatgoogle]

(Merging; PyPy platform test failures are unrelated.)