Fix storyboard example

Fixes https://github.com/John-Lluch/SWRevealViewController/issues/268
two major changes here:
- the VC is switched in the existing navController so the perform block no longer allocates a new NavController on every menu selection  
- prepareForSegue checks for the view controller being "reselected" and dismisses the menu in this case
There are also some minor changes to the ColorViewController.m to handle this.

I also noticed (after making this change) that you have already fixed this. I'mm still make this all request because I feel this change has _some_ merit, specifically, it doesn't push a new VC when you select the same menu item.