Add UISwitch Category to enable child switch animations

Although the app was working as designed, the child switches were not being animated when using setValue:ForKey on the array from the IBOutletCollection. Add the category to use the setOn:animated method to enable the animation. The category method is called via makeObjectsPerformSelector:withObject method on the array. This is within the rules of the challenge since there is no loop used and the method name does not begin with 'enumerate'.
Replaced int with NSInteger, made switchCounter private, code clean up
Minor edit
Refactor and remove NRDSwitchCalculator class, replace with collection operator

Creating a helper object to calculate simple addition and manage a counter property is a bit of overhead. The use of the simple collection operator @sum on the "on" property of the UISwitches in the IBOutletCollection replaces the purpose of the NRDSwitchCalculator. This provides a significant reduction in code.