Added the CoreFoundation Xcode project.
Added the initial CoreFoundation target files.
Set the framework bundle identifier.
Removed an unnecessary framework from the linkline.
Added preprocessor definitions to the build.
Added the contents of the prefix header.

I also added CFBase.h, which is included by the prefix header.
Added CFAvailability.h.
CFBase.h and CFAvailability.h are now built as public headers.