Updated Riemann output:

* Customizable 'separator' option instead of hard-coded '_'

* String values are sent as "State" instead of "Metric", preventing
  Riemann from rejecting them

* Riemann service name is set to an (ugly) combination of input name &
  (sorted) tags' values...this allows connecting different events for
  the same input together on the Riemann side

closes #642