Sorted dict 1

Pull request for SortedDict and BalancedTree23 taking into account Kevin Squire's feedback from the past 12 days.
Sorry, hit enter before finishing.
```julia
immutable TreeNode{K}
    child::(Int,Int,Int)
    parent::Int
    splitkey::(K,K)
end
```
The main benefit would be that `child` would then be iterable, but accessing it directly should still be fast.  (This should be tested of course!)

This doesn't have to happen right now, but I think it would clean up some code below, especially in `insert!` and `delete!`.