Fixed NodeMixin Example

[StevenLimpert]

I replaced "import Node" with "import NodeMixin" in the NodeMixin example.

This was necessary for me to get the example to work when I first used anytree.

I am new to Python and GitHub, so apologies if it is my mistake and this is not necessary.

Thank you for anytree! It is a very nice package.

[coveralls]

Coverage remained the same at 100.0% when pulling fe48c09 on tyrone-slothrop:NodeMixin_example_fix into 56725db on c0fec0de:master.