An element should only be initialised once.

[treshugart]

When skate.init() is called on an element, it should only allow initialisation once for that element. This will not work:

var element = document.createElement('div');

// first binding
element.setAttribute('data-binding-1', '');

// initialise
skate.init(element);

// second binding
element.setAttribute('data-binding-2');

// does not initialise second binding
skate.init(element);