Leopard compatible

I've run all the testers on Leopard, and the couple of issues I found were actually my fault, so I fixed those and now it looks like the framework is just as functional on Leopard as Tiger. There may be a difference in how invalid attributes are handled on KeychainItems, as I haven't checked exhaustively yet, but I'm reasonably sure its the same.

If you find any Leopard issues, of course do let me know. I'll also need to go through, at some point, and do a proper audit on all the areas that have changed (such as supported algorithms, for example).