State of switches should automatically reflect state of the underlying input's "checked" property

At present modification of the `.checked` property of the underlying input of switches (and perhaps other similar components, e.g. radio and checkboxes, I haven't checked, pun intended) doesn't automatically result in an update to the presentation layer - instead a manual call to `.MaterialCheckbox.check()` or `.MaterialCheckbox.uncheck()` is needed.

I propose that this shouldn't be the case, MDL should progressively enhance the underlying input by listening for changes to the value of `.checked` and automatically fire the appropriate function to update the visual state, which would allow existing code / code from other libraries to be used without modification and would avoid confusing new users of MDL.