Signedness tweaks

Fishhook is awesome, thanks for sharing with everyone!

ReactiveCocoa uses DYLD interposing on OS X to capture asynchronous backtraces, and now with fishhook the same can be done on iOS.

When I integrated fishhook into ReactiveCocoa, a few signed conversion and comparison warnings were reported, but since RAC uses `-Werror`, they become errors.

If you're interested, here are the changes I made to remedy the signed conversion and signed comparison warnings.