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.
@aphyr No Kyle, the timestamp of event is used to generate the datapoint.
https://github.com/pradeepchhetri/riemann/blob/master/src/riemann/stackdriver.clj#L22
This timestamp will be used will storing the datapoint. But,
:timestamp is used by the gateway to disambiguate messages.

Let me know if i am wrong.