HyperlinkedIdentityField uses `lookup_field` kwarg.

According to the [Serializers API Guide][1], `HyperlinkedIdentityField` takes `lookup_field` as a kwarg like the other related fields and the generic views. However, this was not actually implemented.

I also noticed that there aren't any tests using the `lookup_field` kwarg, but I added one for this fix.

[1]: http://django-rest-framework.org/api-guide/serializers.html#hyperlinkedmodelserializer