Re-enable Python3 tests and fixing Py2/Py3 issues in tests.

Partially fixes #85.
Merge pull request #87 from dhermes/re-enable-tox

Re-enable Python3 tests and fixing Py2/Py3 issues in tests.
Supporting client secrets / user flow with a hack.
Adding a script and example creds file to check user implicit auth.

Also fixing bug where SETTINGS was immutable.