Create BST.java

Ordered symbol-table API which uses a binary search tree built from Node objects that each contain a key, associated value, two links, and a node count N. Each Node is the root of a subtree containing N nodes, with its left
link pointing to a Node that is the root of a subtree with smaller keys and it's right link pointing to a Node that is the root of a subtree with larger keys. The instance variable root points to the Node at the root of the BST.