Keep scroll indicator view on top

The fix is present only in the last commit (PSTCollectionView class), please ignore changes from PSTCollectionViewCell.

![iOS Simulator Screen shot Feb 19 2013 4 12 43 PM](https://f.cloud.github.com/assets/1692861/171510/87291146-7a9e-11e2-9adc-d2e88327a236.png)