update com.google.gerrit.extensions

- add methods to RevisionApi (FileApi)
- new JSON entities for files, diffs, etc.

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

- adds methods to extract files and diffs from revisions
- some minor refactoring in building request URLs for changes and revisions
Lazily instantiate REST api clients in GerritApiImpl

The various REST api clients will now only be instantiated
at the time they are first called.