Added a static library build process for conditional linking

I've added an Xcode project which will build a universal static library (based on [this template](https://github.com/michaeltyson/iOS-Universal-Library-Template)), so one can use a conditional linker flag to only link to BugshotKit from Debug/Ad-Hoc builds. That way, no BugshotKit symbols will be present in an App Store build. Instructions added to the Readme.