Create custom duck assertions for chai

Allows you to assert the following:
expect(someDomElement).to.be.visible;
expect(someDomElement).to.be.hidden;
expect(someDomElement).to.be.focused;
expect(someDomElement).to.be.disabled;
expect(someDomElement).to.be.enabled;
expect(someDomElement).to.be.removed;
// also the 'not' version of each, e.g.
expect(someDomElement).to.not.be.enabled;

To use the duck-chai plugin in your tests, write `chai.use(duckChai);`