Add a better "from" field implementation

[nbartels]

The from field in Video, Comment, Post and so on can be a User or a Page, but atm we only provide a CategorizedFacebookType. This is not sufficient in every case, so we need a way to access the page or the user.

Idea: Provide a new From that contains access to the User and the Page type, and it's up to the developer to take the right object.

This should be a general solution for #1134