[WIP] Upgrade to oauthlib 0.6.1 (bug 917040)

https://bugzilla.mozilla.org/show_bug.cgi?id=917040

WIP for those who are familiar with our OAuth code and/or oauthlib. It needs 2-legged tests, which we don't seem to have currently. Also, I have passed `mkt.api.tests.test_oauth` but haven't tried the rest of the API tests yet.