[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/exifdata.cpp

@@ -152,6 +152,9 @@ std::string formatXML(Exiv2::ExifData& exifData)
 ///////////////////////////////////////////////////////////////////////
 int main(int argc,const char* argv[])
 {
+	Exiv2::XmpParser::initialize();
+	::atexit(Exiv2::XmpParser::terminate);
+
 	format_t formats;
 	formats["wolf"] = wolf;
 	formats["csv" ] = csv ;