Allow API-wide overriding of the url field behavior

This PR allows specifying a field name for the hyperlink that will be put on the serializer, instead of assuming it should be `url`.  For backwards compatibility, `url` is still the default, but it can be overridden now by specifying `URL_FIELD_NAME` in the REST_FRAMEWORK settings dict.

The other bit of functionality in this PR is the ability to specify relative urls instead of absolute urls.  This is similarly accomplished via the setting `RELATIVE_URLS` which defaults to `False` for backwards compatibility.