The important things to note in this gist are:
app.html
The usage of the custom attribute hover-color is <div hover-color="special-hover-class"> where special-hover-class is
simply a css class that will be applied to all hover-color elements when the given element is hovered.
hover-color.js
This is the source code of the custom attribute itself, and is commented to explain what it does. Essentially, if the element
is hovered, the custom attribute sends out a message to tell all other hover color elements to change. When the mouse leaves
the element, a message is broadcast to tell them all to remove their class change.