Add fishhook.c

https://github.com/facebook/fishhook becd6c43f4891011676fc283b8b4afaf767747a2
Enable fishhook only for DEBUG
Prefix fishhook API with rac_
Use -Wno-sign-compare and -Wno-sign-conversion for fishhook.c
Enable NSOperationQueue backtrace test on iOS
Integrate fishhook into RACBacktrace

With this change, the clang static analyzer reports false memory leaks on the
iOS target.
Cast away signed conversion warnings
Cast away signed comparison warnings
Test for iOS builds using __IPHONE_OS_VERSION_MIN_REQUIRED
Use C99 designated initializers for fishhook rebindings
Update backtrace documentation for iOS
Remove local fishhook copy
Subtree fishhook

https://github.com/facebook/fishhook becd6c43f4891011676fc283b8b4afaf767747a2
Disable fishhook compiler warnings
Prefix fishhook API with rac_ (subtree redux)
Disable static analyzer for offending false positives on iOS
Fix C string lifting test.
Merge pull request #710 from ReactiveCocoa/fix-cstring-lifting-test

Fix C string lifting test.
Add documentation link to facebook/fishhook
Move rac_dispatch_ functions into RACBacktrace+Private.h

These functions and macros are used internally, in order to override standard
GCD dispatch_ functions to capture async backtraces. This is only necessary on
OS X, where interposing only applies to the app's code (image), not the linked
ReactiveCococa framework.