Refined event service construction

Originally Event.service field contained only plugin name, but that was
not correct since one plugin can report multiple values (e.g. cpu,
memory etc.).

The code ain't most beautiful because different plugins report
measurement description in different way so service field is
constructed from following fields joined with dot:
  - plugin name
  - plugin_instance
  - type_instance
  - data_set[index].name
Merge branch 'riemann-plugin' into debian-package