Add in tests for instance based navigation controller functionality

app/test_screens/navigation_controller.rb

@@ -0,0 +1,3 @@
+class CustomNavigationController < PM::NavigationController
+  
+end

lib/ProMotion/screen/nav_bar_module.rb

@@ -44,7 +44,7 @@ module ProMotion
     def add_nav_bar(args = {})
       self.navigationController ||= begin
         self.first_screen = true if self.respond_to?(:first_screen=)
-        nav = NavigationController.alloc.initWithRootViewController(self)
+        nav = (args[:nav_controller] || NavigationController).alloc.initWithRootViewController(self)
         nav.setModalTransitionStyle(args[:transition_style]) if args[:transition_style]
         nav.setModalPresentationStyle(args[:presentation_style]) if args[:presentation_style]
         nav

spec/unit/screen_spec.rb

@@ -125,14 +125,20 @@ describe "screen properties" do
 
   describe "navigation controller behavior" do
 
-    it "should have a nav bar" do
-      @screen.nav_bar?.should == true
+    it "should let the instance set the nav_controller" do
+      screen = HomeScreen.new nav_bar: true, nav_controller: CustomNavigationController
+      screen.on_load
+      screen.navigationController.should.be.instance_of CustomNavigationController
     end
 
     it "#navigationController should return a navigation controller" do
       @screen.navigationController.should.be.instance_of ProMotion::NavigationController
     end
 
+    it "should have a nav bar" do
+      @screen.nav_bar?.should == true
+    end
+
     it "have a right bar button item" do
       @screen.navigationItem.rightBarButtonItem.should.not == nil
     end
@@ -260,4 +266,5 @@ describe "screen with toolbar" do
     screen.set_toolbar_button([{title: "Testing Toolbar"}], false)
     screen.navigationController.toolbarHidden?.should == false
   end
+
 end