Added omniauth flow + develop branch (Bounty #13)

Hi!

Three things added here:

**1. Omniauth flow fix**: I completed the flow described in Bounty 13 (https://assembly.com/gamamia/bounties/13). E.g. this flow: (1) User clicks sign in with Facebook or Twitter, (2) User is taken to Facebook or Twitter to accept connection with the app, (3) User is taken back to our app and is asked to confirm e-mail (Twitter does not send e-mail back, Facebook does) and enter occupation, (4) The user clicks continue and the information about the user (e-mail, password, occupation, name) and social media identity (id of the user, type and account id on Facebook Twitter) is stored in our database. Finally the user is redirected to the frontpage. The user can therefore currently sign up and in with Facebook, Twitter and just standard e-mail and password.

**2. Dependent destroy Users->Identities:** Added dependent destroy between identities and users => if a user is deleted from our database it also deletes any social media identities related to the user.

**3. Gitflow prep:** Adhering to this wonderful post about gitflow (http://nvie.com/posts/a-successful-git-branching-model/) I've put this in a branch named "develop" which I think we should keep for the remainder of the project. And only merge into the master branch when we're actually releasing new features to a running platform.

Todo: I'll look into in the coming days to actually setup some tests for this functionality. But since the functionality still is quite simple ;) I think it can wait until a later commit.