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.