Issue #3: Reworked models

Added new migrations for:
 + diashows:
  + added polymorphic parent - (id, type) -  (right now eiter post or
category).
  + removed category_id, post_id columns.
  + Added inverse of associations, belongs to parent
 + posts:
  + has one diashow, which is created as an after save event (only once)
 + category:
  + tweaked associations.

Those migrations will break the existion controller action method code.

What's up next:
 + Fix controllers.
Issue #3: Refactoring Posts

Refactoring code base of post model and posts controller.
Issue #3: Some diashow refactoring

Initial refactoring of diashow controller:
 + simplification of code - removed some code duplications.
 + introduced some private methods.
 + fixed some program logic.

Addes some comments to PostController for later/further refactoring -
where to export the considered piece of code.
Issue #3: Modified routes

Removed unused routes - rank_up, rank_down
Not anymore required since there exists now a update_rank method which
requires a direction which does basically the same trick.
Issue #3: Refactoring create actions

Fixed create actions of categories and posts controller - regarding
diashow.
Issue #3: Refactoring Diashow

Fixed some bugs resulting due to previous refactoring:
 + Removed some unnecessary 'nil' checks.
 + Fixed a syntax error within diashow view regarding diashow_path
 + Fixed diashow update sucht that:
  + new photos can be added to a diashow (for posts and categories)
  + photos can be deleted.
 + Fixed an update bug, which occureed when there was no diashow photo
chosen and someone pressed onto the 'upload photo' button and crashed
the whole engine.
+ Fixed some id and identifier bugs.
Issue #3: Fixed diashow rank edit

Rank is now correclty updated (again) when someone pressed onto either
the 'rank_down' or or the 'rank_down' button.
Issue #3: Fixed photo deletion bug

Since there was some internal renaming happening (since wrong id had
been assigned and have been violated as a place holder - and this for
sure had to be fixed) there were some 'path' bugs regarding deletion of
diashow's photos. This has been fixed.
Issue #3: Refactoring post

+ Fixed post creation: posts again can be created.
+ Fixed post deletion.
+ Fixed post edit.
+ Fixed a routing issue regarding post creation.
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'.
Issue #3: Display post photos

Photos of a post are now displayed in the 'public_show' view if a
category.
Issue #3: Exported code to helpers

Moved some private controller code to their helpers.
Issue #3: Fixed a broken test

A post's title may be blank (from now on - handle this case within
view).
Issue #3: Fixed broken 'back' button

There was a routing bug regarding the 'back' button for post-diashows.
It was still assuming that we only have diashows for categories and
therefore a wrong id was passed for this route.
Issue #3: Fixed a important bug

Added a filter which deletes invalide posts (regarding public show).
Issue #3: public_show_renderer fix

Updated this helper such that it behaves like
'Fadenblog::Categories.public_show action'.

What's up next:
 + Fix missing layout bug.