separates GerritRestClient into GerritApiImpl and GerritRestClient

- GerritApiImpl implements GerritApi interface and gives access to
  endpoints provided by gerrit rest interface
- Renamed GerritRestClientFactory to GerritApiFactory and made its
  #create methods return GerritApi instead of an implementation thereof
- GerritRestClient does not implement GerritApi (anymore) but provides
  basic http access to the gerrit rest interface

 Related to issue #56
add test class for ChangesRestClient

- added tests for generating url in #query methods

Related to issue #56