Separate oauthlib integration into its own package (#137)

* Centralize the run_pylint script.
* Use io.open instead of open in setup.py for httplib2_transport
* Move httplib2_transport -> additional_packages/google_auth_httplib2
* Remove unneeded dependencies in google_auth_httplib2
* Update classifiers to note 3.6 support.
* Don't install the HEAD version of google-auth when testing google-auth-httplib2.
* Add google-auth-oauthlib package.
* Remove google.oauth2.oauthlib and google.oauth2.flow and associated tests.
* Make travis run google-auth-oauthlib's tox.
* Specify tox workdir.
diff --git a/tox.ini b/tox.ini
index 528806c..27f7d87 100644
--- a/tox.ini
+++ b/tox.ini
@@ -77,7 +77,9 @@
     --import-order-style=google \
     --application-import-names="google,tests,system_tests" \
     google tests
-  python {toxinidir}/scripts/run_pylint.py
+  python {toxinidir}/scripts/run_pylint.py \
+    --library-filesets google \
+    --test-filesets tests system_tests
 deps =
   flake8
   flake8-import-order