Depth first search #1

 - Recursive iteration, not especially efficient...
Depth-first search (for binary tree)

 - Uses recursion properly!
Generator iteration for BinaryTree
Cleaning up iteration & insert for BTree
Testing helpers for btree, insertion/iteration cleanup
Insertion for binary tree finalised

 - Also added indexing -- tree[value] access the node with that VALUE, not the node at that index
Messed around with searching
Add Search for Binary Search Tree
Insertion for BST