Implement service: auth

[robertrossmann]

Backend authentication is one of the most frequent pieces of code that we write over and over again with each new project. Let's stop wasting time on such things and write a service which can verify OAuth tokens from most of the OAuth providers out there.

Must support backend authentication for these providers:

• Facebook
• Google
• Twitter
• Local authentication? (via email + password in local db)

More can be added later as needed.