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.
"Messages with the same timestamp are assumed to be the same (see below)"

Oh, actually this is weirder than I thought, haha. You'll have to ensure that each message gets a distinct timestamp. Probably best to encode a timestamp atom in the stackdriver client and updating it for each message with

```clj
  (let [ts (swap! timestamp #(max (inc %) (riemann.time/unix-time)))]
    ...)
```