Show Selected Tab Images

Bug fix that allows the vertical tab bar to have the option to show selected tab images instead of just the highlighted gradient. This is a standard feature on the UITabBarController and should be supported by FSVerticalTabBarController.

To test, instead of setting the tabBar image directly use the following code:

```
 [tabBarItem setFinishedSelectedImage:[UIImage imageNamed:@"selected_image.png"] withFinishedUnselectedImage:[UIImage imageNamed:@"image.png"]];
```