Lazily instantiate REST api clients in GerritApiImpl

The various REST api clients will now only be instantiated
at the time they are first called.
Merge pull request #9 from forrert/refactor-gerritapiimpl

Lazily instantiate REST api clients in GerritApiImpl
update httpclient to v4.3.5
update gson to v2.3
update guava to v18.0
update testng to v6.8.8

- convert junit imports to testng ones
try to fix travis build by downgrading testng to v6.8.7 again
update com.google.gerrit.extensions

- add more javadoc
- new methods in ChangeApi to get suggested reviewers

these changes are still a work in progress on gerrit-review.googlesource.com