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)

Hm. so the problem right now is that I expect initiateOAuth to be synchronous, which becomes a problem when using chrome.permissions to check, which is async. The place where this is perhaps more of an issue, is that the oauth.webrequest provider checked for valid hosts in our permissions. Do you think this needs to be refactored?