[Refactor] Use data binding library

[rinrinne]

Now event data formatted by JSON is binded to event model by fromJSON(). It means that each event models have binding logic.

If we use data binding library like Jackson, we can get more simple event models like beans or pojo. It would allow us to follow the latest gerrit event model easily.

I created a study project. Please check the below if interested.
https://github.com/rinrinne/bind-event-sample