Prop init fix

This resolves an init ordering issue I experienced when nesting the directive within another.

I found that my value on the slider was already set at init, but that the convenience properties (though present in attrs) where only set later. This can lead to strange state in the underlying widget, eg initing with a max of 1000 and an ngModel value of 1000, might lead to the handle initing at the track start rather than end - after (a little too long!) debugging I found that the widget was initing using the default max (100) and step (1), causing the handle to snap to 0 - later updates on max and step from the $observe don't cause the required refresh in the widget.