Support HTML DOM element object as selector in Scene constructor (useful for unit testing)

Some unit testing frameworks such as Karma create a virtualized, in-memory DOM environment that isn't the real DOM. It would be valuable to be able to pass in an element object as the selector to Famous' Scene constructor. Currently it takes only a string selector which searches the real DOM. Being able to pass in an element object as the selector is crucial to unit testing. It allows for better isolation of components that can be tested in any environment, whether the actual DOM or a virtualized one.