Bind method to custom attribute

Hi Seth,

I extended the example "custom_element_with_custom_attribute_binding" a little
to show how to bind an event method to a custom property 
in addition to binding a value.

I found this convenient and thought it fits with this example.

That it is not possible to reference a method of the model directly in HTML (index.html) made the solution slightly more complicated than I hoped.
It is also not possible to declare and initialize a class member variable to a method reference in one statement.
So I moved the assignment to a constructor (in the Model class). 

In the end it works fine but it's not as elegant as I wished. 
Do you know of any plans to improve that?

Günter