Add implementation of Node.contains that jsdom is missing

Currently zombie does not provide an implementation of Node.contains which is commonly implemented in browsers and documented here:  https://developer.mozilla.org/en-US/docs/Web/API/Node.contains

Ember.js 1.5.1 assumes that Node.contains is provided and so the lack of Node.contains breaks testing Ember.js applications.

It looks like zombie used to provide a Node.contains implementation but somehow it was lost in the 2.0.0 branch.  The original pull request #399.  My commit is the same implementation plus some testcases so hopefully the API will not got dropped again.