Disallow min and max to be equal

Problem/Motivation

It would be nice to have an option to disallow selecting the same value for the min as for the max value.

Proposed resolution

Add a check to the JS to see if min and max are equal. If min uquals max, cancel the slide event.
slide: function (event, ui) {
+ // don't allow mix == max
+ if (ui.values[0] == ui.values[1]) return;
set_slider_handle_labels(ui.values[0], ui.values[1]);
},