Ceiling Node in BST
	
	/**
	 * A binary search tree is given. Find the ceiling value present in the
BST
	 * of a given key.
	 * 
	 * e.g.
	 * 
	 * 			 8
	 * 		  /     \
	 * 	     3       12
	 *      /  \    /  \
	 *     2    6  10  15
	 *         /
	 *        4
	 * 
	 * key - 13 => 15 
	 * key - 4 => 6 
	 * key - 8 => 10
	 * 
	 * */