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
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
Merge remote-tracking branch 'remotes/uwolfer/master'

Conflicts:
	src/main/java/com/urswolfer/gerrit/client/rest/http/changes/ChangeApiRestClient.java
	src/test/java/com/urswolfer/gerrit/client/rest/http/GerritRestClientTest.java
Merge pull request #69 from forrert/refactor-rest-client

Suggested changes to refactoring Gerrit rest api