Issue when trying to open the slide window

Hey there,

Thanks for making this, it's exactly the effect I've been looking for!

I have a problem trying to integrate this with my project. This is my current app structure:
Navigation Controller -> MainViewController

Inside MainViewController I have a button that calls:

```
let vc = self.storyboard?.instantiateViewControllerWithIdentifier("GoalVC") as! GoalViewController
self.navigationController?.pushViewController(vc, animated: true)
```

Now inside this GoalViewController instance, I want to show this slide menu controller from the right. I do this on a tableview cell selection:

```
initFormSheet()
if let mySlideMenuController = self.mySlideMenuController {
mySlideMenuController.openRight()
}
```

This is the initFormSheet function:

```
var mySlideMenuController : SlideMenuController?
func initFormSheet() {
if mySlideMenuController == nil {
let formSheet = self.storyboard?.instantiateViewControllerWithIdentifier("DisposableCategoriesVC") as! UIViewController
mySlideMenuController = SlideMenuController(mainViewController: self, rightMenuViewController: formSheet)
}
}
```

The problem is, when the tableview cell is tapped and the above code is executed, nothing happens. My status bar gets hidden, but I don't see any view controller sliding from the right. Can you help me out here please? Can you tell me what I'm doing wrong?

I don't want to use your library for the root window, instead this is inside of a navigation controller sub window.

Thanks