Added is_dir and is_dir example

examples/is_dir/example.cpp

@@ -0,0 +1,18 @@
+#include "is_dir.h"
+using namespace igl;
+#include <cstdio>
+
+int main(int argc, char * argv[])
+{
+  if(argc <= 1)
+  {
+    printf("USAGE:\n  ./example [path_1] [path_2] ... [path_n]\n");
+    return 1;
+  }
+  // loop over arguments
+  for(int i = 1; i < argc; i++)
+  {
+    printf("is_dir(%s) --> %s\n",argv[i],(is_dir(argv[i])?"TRUE":"FALSE"));
+  }
+  return 0;
+}