Synchronize unmarshallers from sonar-ws-client to sonar-gwt-api

It's a shame to have duplicated code between sonar-ws-client and sonar-gwt-api : problem is that we can't use same classes to parse JSON, but actually code is the same. In order to remove this duplication we can reuse the same principle as for WSUtils. Disadvantage - type safety, but I think that this is not a big problem, since we have unit tests for unmarshallers. In fact it would be better to use emulation of types from org.json.simple for GWT, but this solution is more straightforward.