[BACKLOG-218] Add event bus to the platform and make sure it is available on the DI Server

- leverages on guava's async event bus
- interfaces in pentaho-platform-api and implementation of MonitorService in pentaho-platform-extensions
- shell-plugin 'monitoring-eventbus-plugin' that simply starts up MonitoringService
This class needs unit tests which should be VERY simple given the implementation (the tests should execute each method and verify no exception is thrown).