Added Dropbox support and updated README

I've added OAuth2.0 support for Dropbox.  I also updated the README to better reflect the current state of the code.  I believe most of the README updates are good, but I'm concerned about the portion I changed regarding the session var for storing the `AuthInfo` objects.  The README claims you can access a map of `AuthInfo`s which I'm unable to find in the source.  I've updated it to mention the `Omniauth.currentAuth` var which works, but has the limitation that only one provider can be used at a time.