Implement Buck driven build

Clone or link this plugin to the plugins directory of Gerrit tree
and issue the command:

  buck build plugins/gitblit

The output is created in

  buck-out/gen/plugins/gitblit/gitblit.jar

The project can be imported into the Eclipse IDE:

  ./tools/eclipse/project.py

Change-Id: I0026d8cbdd7a70c0ddca8fb41ac387a2ce73a845