Behave in navigationController

As a solution to the subject discussed in [Issue #8](https://github.com/rnystrom/RNFrostedSidebar/issues/8), where [RNFrostedSidebar](https://github.com/rnystrom/RNFrostedSidebar) is used to push a new viewController in a navigationController.

Show the sidebar on top of the navigationController:
```
    RNFrostedSidebar *callout = [[RNFrostedSidebar alloc] initWithImages:images];
    callout.delegate = self;
    [callout show];
```

In `didTapItemAtIndex` call `dismissInNavigation` before sergue to correctly remove the sidebar from the parentViewController before pushing the new viewController, and thereby keep the navigationBar.
```
- (void)sidebar:(RNFrostedSidebar *)sidebar didTapItemAtIndex:(NSUInteger)index {
    if (index == 0) {
        [sidebar dismissInNavigation];
        [self performSegueWithIdentifier:@"newSegue" sender:self];
    }
}
```

You could also just call `[self removeFromParentViewController]` every time in `dismissAnimated:(BOOL)animated`, but I think a dedicated call is more suited.

I think a lot of people is using your control with a navigationController. A test case of this scenario would be nice in [your](https://github.com/rnystrom) demo project, but that is up to you.