Authentication process logging

Hello.

I am providing this pull request not for merge, but for informational purposes only. I wanted to show you what kind of work I had to do to track down a problem with auth.

Some users of our app kept complaining that they could not sign in. But when we checked our logs we realised that we couldn't get any useful information from them because, well, Devise doesn't write any. It just does its magic. Silently.

So I made this branch from the `v2.1.0` tag (the version we're using), pointed the `devise` gem in the `Gemfile` to it, and added a whole bunch of logging there. It worked fine in production and helped narrow the problem.

Could you consider adding logging like that to `master`? Should I rebase what I did from master, polish it, and send another pull request? Please let me know what you think about this whole logging problem.

Thank you.