btree simple

Here's a simple BTree implementation that I modified based on Teepark's BTree.

Current flaws with the BTree:
1. cannot do {key, value} search yet
2. not sure if the remove method should do nothing if an item is not in list OR raise an exception
3. no short way of converting a btree from one order to another

However, I think we can build on this to implement the backend index.