More warnings and one more analyzer flag. At least some of these are new in Xcode 5.

- Warn when a block implicitly retains self.
- Warn when a message is sent to a weak reference.
- Warn when a weak reference is repeatedly used.
- Analyzer: Complain about uses of rand(), random(), etc. (in favor of arc4random).
OK, these two are nice in theory, but currently way too noisy to be useful.
Added -Wunreachable-code, although it doesn’t seem to do anything.