title: Circular dependency in serializers
body: <p>I play with django-rest-framework and I would do following:</p>
  
  <pre><code>from rest_framework import serializers
  
  from .models import Author, Book
  
  
  class BookSerializer(serializers.ModelSerializer):
      author = AuthorSerializer(many=False)
  
      class Meta:
          model = Book
          fields = ('slug', 'name')
  
  
  class AuthorSerializer(serializers.ModelSerializer):
      books = BookSerializer(many=True)
  
      class Meta:
          model = Author
          fields = ('slug', 'name', 'books')
  </code></pre>
  
  <p>But it fails.</p>
  
  <pre><code>NameError at /api/books/authors/
  name 'AuthorSerializer' is not defined
  </code></pre>
  
  <p>Anybody helps?</p>