title: ARC properties' new attributes
body: <p>I've read the transitioning to ARC notes and I still have a bit of confusion about properties' attributes we shoul/can use... We can use weak in place of assign (with the advantage that the property is set to nil if the object it points to is released), strong in place of retain and what in place of copy? We still use copy alone or do we need to couple strong with copy, such as <code>property (strong,copy)</code>.. Maybe I need to practice and read again the doc because ARC is not very clear to me...</p>