Added an outlet collection

Hey Ionut,

It turns out that objects are not retained by default: https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/LoadingResources/CocoaNibs/CocoaNibs.html#//apple_ref/doc/uid/10000051i-CH4-SW6

So I added an outlet collection, and all custom objects should be in that collection.