Spinner: arrow keys support / up-down buttons behaviour

1. arrow keys support: increment or decrement values using up/down arrows, like the native jQuery spinner.
2. up/down buttons shouldn't have tab index: when we are in a spinner text input and press tab, the focus is going to the spinner buttons, but this is not the same behavior of native jQuery spinner. To me, these buttons are useful only for mouse/touch/pointer, and not to keyboard, impairing the overall usability.

With these 2 changes, this plugin will act more similar to the native jQuery spinner.