Use actual value of attribute refresh-on as event name…

…instead of using `$scope.$eval(attr.refreshOn)`.

I was expecting to be able to do

    <div bindonce refresh-on="refreshThings">…</div>

and then

    $scope.$broadcast('refreshThings')

But the way it's currently written, I need a scope variable  called `refreshThings` whose value defines the name of the event. Is that how it's supposed to work? Maybe I misunderstood your intentions, but the proposed change makes more sense to me.