Add proper main entry point to avoid execution when import.

tools/mdexport_test.py

@@ -52,6 +52,7 @@ MDIMPORT = {
 }
 
 
+def main():
     item = MDIMPORT[sys.argv[1]]
 
     metad = None
@@ -66,3 +67,6 @@ if metad:
         metad.load()
         print metad.dumps()
 
+
+if __name__ == '__main__':
+    main()