add disable-with support for standalone remote buttons

These are my changes for allowing a button[data-remote] to have data-disable-with attribute.  They only apply to remote buttons.  I assume that non-remote buttons are handled by the form handlers.  The changes are not quite as DRY as they could be but they are pretty straightforward and not very intrusive.   I'm not quite sure why rails button_to doesn't have an option to create a button without a wrapper, but that would be the next step.  Tests are included.