Generalize user authentication

[afcapel]

Right now, you need an Spanish electronic identity card to vote or register in the application. To make the application more general we need a general system with pluggable authentication modules. We also need some configuration options to setup in each instance which authentication methods will be used.

[afcapel]

Given the requirements we have, I think something like OmniAuth would be handy.

https://github.com/intridea/omniauth

It general framework with support for multiple authentication strategies.

[bglusman]

Posted a comment on 3.1 upgrade, but tractis has some incompatibilities there so I may switch to working on this and then try 3.1 upgrade on omniauth branch, not sure on right strategy there but thought I'd mention here tooo...