Tweaks to allow headers to be glued together.

Added Python script to generate single header.
Added new XCode project that runs self test against single header

include/internal/catch_objc.hpp

@@ -16,6 +16,9 @@
 #import <objc/runtime.h>
 #include <string>
 
+// NB. Any general catch headers included here must be included
+// in catch.hpp first to make sure they are included by the single
+// header for non obj-usage
 #include "internal/catch_test_case_info.hpp"
 
 ///////////////////////////////////////////////////////////////////////////////