Add parse() and traversal methods, updated size()

public static BinaryTree<String> parse(String treeDescription)
Translates a String description of a tree into a Tree<String> object.
The treeDescription has the form value(child child), where
a value is any sequence of characters not containing parentheses or
whitespace, and each child is either just a (String) value or is
another treeDescription. Whitespace is used to separate values.
For example, the String "one(two three(four(six seven) five)) six"
becomes the Tree:
                            one
                            / \
                           /   \
                         two  three
                         		 /\
                         		/  \
                            four five
                             /\
                            /  \
                          six seven
Also, added a private helper method which tokenizes the given string
so that it can be split into two separate child strings out of which
two separate BinaryTree nodes can be made

public void inOrderTraversal(), preOrderTraversal(), postOrderTraversal()