Completed cosing of substring.

testing/zstring_test.cpp

@@ -85,6 +85,11 @@ int main(int argc, char **argv) {
    coreutils::ZString test14("25.5");
    coreutils::ZString test15("xyx");
    std::cout << test14.startsWithDouble() << ":" << test15.startsWithDouble() << std::endl;
-   
+
+   coreutils::ZString test16("This is a test of substring");
+   std::cout << test16.substring(10) << std::endl;
+   std::cout << test16.substring(10, 4) << std::endl;
+   std::cout << test16.substring(0, 7) << std::endl;
+   std::cout << test16.substring(18, 20) << std::endl;
    
 }