Issue #3: Enhancement Posts

Each fadenblog post has now a own diashow which is initially empty. This
Diashow will be created as an aftersave event (after a fadenblog post
has been successfully created). It also will be cascaded deleted
whenever we are going to delete a fadenblog post.

+ Added link to a post's diashow in category show view page.
+ Tweaked code such that it behaves polymorphic - regarding diashow
functionality.

What's up next:
 + display/render 'photos' from each post in 'public_show method'.