Provider configuration updated and extended in documentation

Elaborated upon configuring Google and Twitter and now mention review proces for Facebook apps.

Note: second attempt, first pull request, travis build failed on unrelated (?)  error.
Digging on https://dev.twitter.com/oauth/reference/post/oauth/request_token section "Example Result", found that the correct config is:

* Client id, is called "Consumer Key (API Key)" on Twitter
* Secret key, is called "Consumer Secret (API Secret)" on Twitter
* Key, is not needed, leave blank

At least is what Is working here.