Sorted dict 1

Pull request for SortedDict and BalancedTree23 taking into account Kevin Squire's feedback from the past 12 days.
Since TreeNodes are immutable, it would be worth exploring changing `child1`, `child2`, and `child3` to a three tuple of type `(Int,Int,Int)`.  

```julia
immutable TreeNode{K}
    child::(Int,Int,Int)
    parent::Int