included_serializers may be specified with strings

[leifurhauks]

in the included_serializers class attribute, classes may be specified as a
string containing the absolute python path for the desired class. Thus, the
values in included_serializers may be a class, a string with the python path
of the desired class, or 'self' to specify the class of the current serializer.

for example:

from rest_framework_json_api import serializers
from rest_framework_json_api.relations import ResourceRelatedField

from myapp.models import Thing
from anotherapp.serializers import CategorySerializer

class ThingSerializer(serializers.HyperlinkedModelSerializer):
    class Meta:
        model = Thing
        serializer_related_field = ResourceRelatedField

    included_serializers = {
        'category': CategorySerializer,
        'accessories': 'myapp.serializers.AccessorySerializer',
        'related_things': 'self'  # ThingSerializer
    }