Improved disabled state once again

Hey Jonathan,

Thanks for the quick acceptance of my previous PRs.

I've moved the `disabled` option to `options`, since this is common in jQM (e.g., in [textinput.js](https://github.com/jquery/jquery-mobile/blob/master/js/widgets/forms/textinput.js#L54)).

Furthermore I re-added support for disabling a spinbox before creation, which seems to got lost while merging my previous [commit](https://github.com/jtsage/jquery-mobile-spinbox/commit/da70d239d9bc434dd4b596b150fe674654401488). (Or did you remove it on purpose?)

Finally I made some changes the disable function which also probably [got lost](https://github.com/jtsage/jquery-mobile-spinbox/commit/1b8c878acaa96c534bc25dc4ac5bfa86a17bf79a).

Cheers,
Jonathan