Refactor Changes Endpoint

- split comments parsing in RevisionApiRestClient into separate class
- add unit test for CommentsParser

Note: I am starting to feel a bit weird about instantiating all these parsers in GerritApiImpl and passing them down to the respective REST clients. I think it might be a little nicer to have access to all parsers through a single object that can be passed to all REST clients (maybe a ParserProvider?). Same goes for passing REST clients down to other REST clients. I don't really have an idea right now though about how to fix this. Maybe it's not such a big deal after all... It is certainly a nicer setup to write tests than having static access to these services. What do you think?