Bug fixes

Ran into problems while trying to get GSDownloadTracker.m to compile with Xcode 4.6.3 for an iOS 6.0+ application. We're compiling with LLVM v4.2 under Mac OS 10.8.4 (Mountain Lion). 

In this environment, the include of `<net/if.h>` generates errors because `struct sockaddr` and `struct sockaddr_storage` haven't been defined. Including `<sys/socket.h>` earlier seems to fix this problem.

Additionally, the LLVM Static Analyzer highlights a potential memory leak in `hashedMACAddress`