Added stackdriver plugin.

Please let me know if i should change something. I tried my best. I tested it by pushing some riemann-health related metrics to stackdriver as well by creating a trial account.
This doesn't need to be runtime extensible, so there's no need for defmethod here. Can also remove duplicate code by folding both generators into the same codepath. Docstring would also be a good idea.

```clj
(defn generate-datapoint
  [opts event-or-events]
  (let [events (if (sequential? event-or-events) events (list event))]
    (map (fn [event]
      {:name (metric-name opts event)
       :value (or (:metric event) 0)
       :collected_at (long (:time event))}))
      events))
```