Refactor return of renderer.convert_resource

Was tuple of item, links, and linked dicts
Now a dict with these expected keys:

- 'data': dict of plain attributes
- 'linked_ids': dict of IDs of linked resources
- 'links': dict of links
- 'linked': dict of linked resources
- 'meta': dict of meta attributes
Refactor renderers.convert_resource into loops

For each field in resource, looks for a converter in
renderers.convert_by_name and .convert_by_type.  If no converter, then
just copy the field.
Merge pull request #14 from jwhitlock/refactor_convert_resource

Refactor renderers.convert_resource