Implement tests for labItem
Add explanatory comments to LabeledTree.hs
Instance Tree as a Functor
Reimplement object tree

/World.hs
- we now organize the objects of the game in an object tree.  This tree
  is a LabeledTree
- reimplemented the type classes of the world object to fit the new
  object tree

/Utils/LabeledTree.hs
- we added a new function for searching an indexed Item from the labeled
  tree and replace it by another value