Restructure YAML package suitable for pub lish

pkg/yaml/lib/yaml.dart

 // Copyright (c) 2012, 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.
 
+/// A parser for [YAML](http://www.yaml.org/).
+///
+/// Use [loadYaml] to load a single document, or [loadYamlStream] to load a
+/// stream of documents. For example:
+///
+///     import 'package:yaml/yaml.dart';
+///     main() {
+///       var doc = loadYaml("YAML: YAML Ain't Markup Language");
+///       print(doc['YAML']);
+///     }
+///
+/// This library currently doesn't support dumping to YAML. You should use
+/// `stringify` from `dart:json` instead:
+///
+///     import 'dart:json' as json;
+///     import 'package:yaml/yaml.dart';
+///     main() {
+///       var doc = loadYaml("YAML: YAML Ain't Markup Language");
+///       print(json.stringify(doc));
+///     }
 library yaml;
 
 import 'dart:math' as Math;
 import 'dart:collection' show Queue;
 
 import 'deep_equals.dart';
 
 part 'yaml_map.dart';
 part 'model.dart';
 part 'parser.dart';
@@ skipping 31 matching lines @@
 }
 
 /// An error thrown by the YAML processor.
 class YamlException implements Exception {
   String msg;
 
   YamlException(this.msg);
 
   String toString() => msg;
 }