A better way of handling offscreen table view

[radianttap]

This expands on the "nicer implementation" comment in the original article and gives few code examples. I have also linked my UICV+NSFRC category that handles onscreen/offscreen updating in the correct way, with no additional code or checks needed anywhere in the code.

[floriankugler]

@chriseidhof mind reviewing this?

[chriseidhof]

I had a look at it, but I think it is too much about your library. We usually try to not promote libraries like this. I think the first half of the changes doesn't really help (it shows which methods to implement)...