Added asDouble method to ZString.

testing/zstring_test.cpp

@@ -4,6 +4,12 @@
 
 int main(int argc, char **argv) {
 
+   // Test floating point reader.
+
+   coreutils::ZString pi("3.14159");
+   double x = pi.asDouble();
+   std::cout << "pi: " << x << std::endl;
+   
    // Test comparison operations.
 
    coreutils::ZString test1("else");