Feature/save password hashes

Here is the pull request, this is really minimal and uses [has_secure_password](http://api.rubyonrails.org/classes/ActiveModel/SecurePassword/ClassMethods.html#method-i-has_secure_password) without password presence and confirmation validations.

The migration should work fine (tested locally) and will save password hashes for existing users. Other than that it is not reversible because we obviously cannot get the passwords back.