P56 - Symmetric binary trees.

Let us call a binary tree symmetric if you can draw a vertical line through the root node and then the right subtree is the mirror image of the left subtree.  Add an isSymmetric method to the Tree class to check whether a given binary tree is symmetric. 

Hint: Write an isMirrorOf method first to check whether one tree is the mirror image of another.  We are only interested in the structure, not in the contents of the nodes.