OAuth 2.0

Major refactoring for the new authentication flow based on OAuth 2.0 (instead of OAuth 1.0).

_c.f._
https://groups.google.com/forum/#!topic/mendeley-open-api-developers/KmUQW9I0ST0
https://sites.google.com/site/mendeleyapi/home/authentication

We plan to merge this code to the master branch by April 14, 2014.