Fix missing jackson annotations for enum

[mcassidygamma]

I encountered an issue when using the jaxrs-spec generator. For a pojo which used an enum, the enum was trying to use @JsonCreator and @JsonValue which were never being imported, thus compilation would fail.

I've added the imports and added a test also. Resubmitting after grabbing all the most recent master branch changes.

[blaukvik]

This would be nice - Is there a SNAPSHOT of maven-plugin with this somewhere ? I can test :-)