Changing onClick so that the implementation can control if a click succeeds

[mraymond]

By moving the onClick event call up and checking the return value, it enables users to prevent the box from being checked. This opens up new features, such as limiting the amount of boxes that can be checked, requested in #194

var count = 0;
var max = 2;
$("select").multipleSelect({
  onClick: function(view) {
    if(!view.checked && count+1 > max) {
      return false;
    }
    if(!view.checked) count++;
    else count--;
});

[ghiscoding]

I think this PR can be closed since onBeforeClick was added in PR #610 which can be used for this exact use case