Add joining of multiple accounts

I added the ability to join multiple accounts.
The saveOrUpdateUser function creates a new user or joins the new data to the existing user.
This Pull Request has been tested with all implemented providers (Twitter, Facebook, GitHub, Google, Linkedin).
For testing purpose I added a new view (account.html). There you can join new accounts and see which accounts are already joined.
![screenshot 2014-03-14 22 37 25](https://f.cloud.github.com/assets/2142829/2426150/eb0e64f0-abc0-11e3-8dd2-2636b72a1a21.png)