handle GenericReferenceField well + organize imports
Fix + test for related_resources
remove print
Allow returning extra data in Resource.get_objects + test
Serialize None properly in callable related_resources
Adding Resource.get_requested_fields which is designed to customize returned fields per request
Making the default _limit configurable
fields fix
Ability to return optional fields (?_fields=_all returns all of them)
value_for_field returns values for fields that don't exist in the resource or the object
Use get_requested_fields for fetch_related_resources in get_objects to use the same fields parsing logic.
If we reraise UnknownKeyError in value_for_field, don't include the field in the response.
handle renamed fields properly for _order_by
return json errors instead of raising werkzeug exceptions
handle NotFound properly
Merge pull request #54 from elasticsales/respect-accept

Proper formatting of error responses
added delayed app initialization
worked out delayed call to app.add_url_rule
fixed eager app init
fixed method permissions + unit tests + minor code cleanup