Updated readme file to include the Api annotation import

This should make it easier for users to jump right in instead of having
to find the proper import first.
Renamed the annotation from Api to JsonApi
Renamed the plugin to Json Apis (removed the word 'variants').
Fixed a problem where wrong properties would get serialized.

The annotated properties code did not distinguish between a properties
not having any annotations set and properties having the catch-all
@JsonApi() annotation defined.
Added a log statement to help debug Json namespace configurations.