refactor ChangesRestClient

- extract json-parsing part into ChangesParser
- add unit test for ChangesParser
- fix bug in DateDeserializer (wrong pattern for date parsing)

Related to issue #56
refactor ProjectsRestClient

- extract json-parsing part into ProjectsParser
- add unit test for ProjectsRestClient and ProjectsParser

Related to issue #56
implement ChangeApiRestClient#addReviewer

Related to issue #56