Change executable name to "darfmt". Fix #163.

bin/format.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:args/args.dart';
 import 'package:dart_style/src/dart_formatter.dart';
 import 'package:dart_style/src/formatter_exception.dart';
@@ skipping 142 matching lines @@
   var output = stdout;
 
   var message = "Reformats whitespace in Dart source files.";
   if (error != null) {
     message = error;
     output = stdout;
   }
 
   output.write("""$message
 
-Usage: dartformat [-n|-w] [files or directories...]
+Usage: dartfmt [-n|-w] [files or directories...]
 
 ${parser.usage}
 """);
 }