Set the tintColor in the auth view and modal views navigation controller on iOS 7

- The authentication controller now sets the buttons tintColor always to the default blue color. This fixes white UIWindow tintColor, since the auth view is using white as background.
- Navigation controllers for modal views now also set the tintColor of the navigationBar to the default blue color on iOS 7, if the developer did not set the `navigationBarTintColor`. This fixes white UIWindow tintColor for feedback and beta update modal views as they are defined to use the default style.

Classes/BITAuthenticationViewController.m

@@ -122,6 +122,9 @@
                               buttonSize.width,
                               buttonSize.height);
     button.autoresizingMask = UIViewAutoresizingFlexibleBottomMargin | UIViewAutoresizingFlexibleTopMargin | UIViewAutoresizingFlexibleLeftMargin | UIViewAutoresizingFlexibleRightMargin;
+    if ([UIButton instancesRespondToSelector:(NSSelectorFromString(@"setTintColor:"))]) {
+      [button setTitleColor:BIT_RGBCOLOR(0, 122, 255) forState:UIControlStateNormal];
+    }
     [containerView addSubview:button];
     [button addTarget:self
                action:@selector(handleWebLoginButton:)