Custom margin and Nib file support

Hi, I'm going to use the PSCollectionView in my project, I really like your works and I made some very small changes that I hope you will find useful too.

1. I've added a property to customize the margin between cell,  the default margin still the one defined into the class implemetation

2. I added the IBOutlet type to the collectionViewDelegate and collectionViewDataSource to be easily connected to from interface builder, and I have also implemented the 'awakeFromNib' method to make the PSCollectionView works when is loaded from a Nib file

Bye 

Manuele Maggi