Add is_symmetric property

[gillespiecd]

Background

I enjoy using binarytree, especially find printing useful in ipython.

I thought it would be fun to add a new property is_symmetric to properties. This comes up sometimes as an exercise with trees -- is it symmetric (a mirror of itself).

Also wanted to learn more about the doctest and coverage modules 👍

Updates

1. Add new property and function to calculate if binary tree is symmetric

Steps to Reproduce

>>> from binarytree import Node >>> a = Node(5) >>> a.is_symmetric True >>> a.left = Node(6) >>> a.is_symmetric False >>> a.right = Node(6) >>> a.is_symmetric True >>> a.right.left = Node(7) >>> a.is_symmetric False >>> a.right.right = Node(8) >>> a.left.left = Node(8) >>> a.left.right = Node(7) >>> print(a) __5__ / \ 6 6 / \ / \ 8 7 7 8 >>> a.is_symmetric True 

[coveralls]

Coverage remained the same at 100.0% when pulling 3523749 on gillespiecd:symmetric into 23cb6f1 on joowani:dev.

[coveralls]

Coverage remained the same at 100.0% when pulling c0a796d on gillespiecd:symmetric into 23cb6f1 on joowani:dev.

[joowani]

Hi @gillespiecd,

Looks good! Thank you for your interest and contribution :)