add search and delete in binary-search-tree.js

add search and delete method in binary search tree in JavaScript.
reference of delete: http://www.algolist.net/Data_structures/Binary_search_tree/Removal

Also I did a few change on add method: if add a existed one, it add nothing and still return root instead of false, so it will not break the chain call.

search return node.

delete return root.