Added an offset parameter to the popup view

As I described on issue #9.
I decided to examine the code and turns out it was quite simple to implement.
Here's an usage example, modified from the original code example.
```Objective-c
SamplePopupViewController *samplePopupViewController = [[SamplePopupViewController alloc] initWithNibName:@"SamplePopupViewController" bundle:nil];

// Add the following line to change the offset
[samplePopupViewController setPopupViewOffset:CGPointMake(0, -40)];

[self presentPopupViewController:samplePopupViewController animated:YES completion:^(void) {
    NSLog(@"popup view presented");
}];
```

If no offset is set, nothing will change and the view will be centered on screen.