Add class SerializerLambdaField that works like SerializerMethodField

Example :

That allows : 

``` python
class ArticleSerializer(serializers.ModelSerializer):
    short_title = serializers.SerializerLambdaField(lambda obj: obj.title[0:10])

    class Meta:
        model = Article
        fields = ('id', 'title', 'short_title')
```

instead of

``` python
class ArticleSerializer(serializers.ModelSerializer):
    short_title = serializers.SerializerMethodField('get_short_title')

    def get_short_title(self, obj):
        return obj.title[0:10]

    class Meta:
        model = Article
        fields = ('id', 'title', 'short_title')
```