They've stated it in interviews. If you want proof, go find it yourself. Don't be a fucking idiot and sit on here saying show me show me!

Also, on the have to kill bosses to get credit, I'd like that idea. Play with a few friends. Everybody get to the boss and kill it. Then new game. Everybody get to the boss and kill it (while the person who got the kill last time sits out). This even works for Act2 as long as everybody drops when somebody opens the tomb then rejoins (so they don't lose staves). The reason I like this is that it has been stated that act bosses will drop much much better the first time and not so well after that. So... with a group of say 5 friends, your group will get to kill each act boss for the first time 5 times.

What is it they say about silence being golden?