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 :)