Rework onPublishComplete

[jonaskello]

Today a callback can be sent in for onPublishComplete. In relation to #36 this naming can be confusing as there is no "complete" state in the standard.

However someone that uses the standard could decide that eg. when root.json has no missing dependencies then it is complete. But this decision should be for the application, not something implementing the standard.

So the callback should probably be named something else and be more generic. Perhaps onFilePublished or something like that.