Fixes spacing issues (visible when all items in the list have identical width)

When using a list of strings that have the same size, the spacing between the items is zero (ignoring the `interitemSpacing` property) and as a consequence the selected item is off-centered (see screenshot, which was produced with `interitemSpacing=20`).

To reproduce, just replace the titles array in the sample demo with a list of identical-width strings, e.g replace

```
self.titles = @[@"Tokyo",@"Kanagawa",@"Osaka",@"Aichi"...];
```
with
```
self.titles = @[@"00:00",@"00:10",@"00:20",@"00:30"...];
```


Note: also removed the empty `AKPickerViewSampleTests ` target that tripped my compiler


![screen shot 2014-10-21 at 8 50 35 pm](https://cloud.githubusercontent.com/assets/150367/4730976/aa1c502e-59a3-11e4-83b2-a619ca0e3009.png)