Collection and CollectionField added

One can name and save a collection. Collection can have many elements
stored in text database field. Every element of this collection may be
deleted.(THIS DOESNT YEAT WORK PROPERLY, ITS GONNA DELETE ALL ELEMENT
THAT MATCH THE ARGUMENT IN DELETE METHOD!). Later this Category may get
Field with 'collection' kind and specific Collection object, which will
be used by the User while making a Poster.
Bottom lines, it allows admin to create fields which has to be filled
based on same collection of elements