Added startsWithDouble() method on ZString.

testing/zstring_test.cpp

@@ -81,5 +81,10 @@ int main(int argc, char **argv) {
    coreutils::ZString test12("1");
    coreutils::ZString test13("12");
    std::cout << (test12 < test13) << ":" << (test13 < test12) << std::endl;   
+
+   coreutils::ZString test14("25.5");
+   coreutils::ZString test15("xyx");
+   std::cout << test14.startsWithDouble() << ":" << test15.startsWithDouble() << std::endl;
+   
    
 }