[xmpsdk] Add XMP Initialisation/Cleanup code to all main() programs

- update documentation
- Removed redundant calls to Exiv2::XmpParser::terminate() (legacy of our
  discusssion with @tester0077).
- this fixes #976

samples/xmpsample.cpp

@@ -19,6 +19,9 @@ bool isEqual(float a, float b)
 
 int main()
 try {
+    Exiv2::XmpParser::initialize();
+    ::atexit(Exiv2::XmpParser::terminate);
+
     // The XMP property container
     Exiv2::XmpData xmpData;