refactor core.oauth

compatibility with new freedom v0.6 core.oauth API

2 oauth providers introduced based on chrome.identity and chrome.webRequest.
oauth.identity passes the integration test, but no testing setup for oauth.webrequest yet 
(chrome.webrequest only available on Chrome extensions and current selenium test is a packaged app)

You can do that in the global scope, like
https://github.com/freedomjs/freedom-for-chrome/commit/fd1924835e6f4c1766fa6e24157096d62e2ba366#diff-e39708bdeb9f4c172aa9c7f80419c20dR13