* Add support to nullify relationship; http://jsonapi.org/format/#document-resource-object-linkage
* Fixed: [null] is not valid as an empty relationship.
* add support for 'omitempty' on relationships; default behavior of marshalling empty/nil relations (i.e. w/o 'omitempty' tag) marshals with null data relation
* cleanup whitespace
* Added go 1.7 to test versions; fixed the marshaling of empty relations to return an empty array rather than a null/nil. Added a more robust test case for the marshaling of non omitted relations.
* Cleanup.
* Added a comment to UnmarshalMany
* Document the ‘omitempty’ annotation on a relation.
* Add common JSON API values as exported jsonapi pkg constants.
Aren Patel