Destroy method for Kalendae.Input which removes all event handlers

I've had issues with memory leaks when creating and removing many Kalendae instances on a single page. Particularly the `mousedown` event handler that is added to `window.document` always leaves a reference to the original object, retaining it in memory until page reload. This patch adds a `destroy` method which simply removes these handlers.

Also: sometimes `window.getComputedStyle` returns null. I've added a few lines to prevent this from breaking Kalendae by letting `getStyle` return an empty string if that happens.