refactored rex_article/rex_category
removed rex_category::isAncestor()
refactored rex_media
refactored rex_media_category
added instance pool trait
simplified article cache
improved instance pool, use arrays instead of dynamic params
instance pool: caching null values
added instance list pool trait
fixed addInstanceList(), made getInstanceList() "protected"
added comments, fixed typo
use array_key_exists
removeInstance(List) -> clearInstance(List)
return null for $id<=0
removed clang parameter of getChildren and getParent
return empty array if $parentId<0
fixed utf8 issues, use english comments
simplified article/category id check
rex_media::get(): sanitize $name
added tests