tool: add "address" command to print D-Bus address

This would be helpful to debug with dbus-monitor.  Instead of:

  $ . .config/ibus/bus/*
  $ dbus-monitor --address $IBUS_ADDRESS ...

one can do:

  $ dbus-monitor --address $(ibus address) ...

This patch also adds help message to each sub-command.

BUG=

Review URL: https://codereview.appspot.com/11901044