Exponentially backed-off respawns with maximum attempts

I find this useful when used in concert with minExpectedLifetime: '0s'

Adds 3 new options:
-backoffRespawns: null to disable; -1 to enable with no max. attempts; positive sets maximum attempts
-backoffDelay: node-backoff "initialDelay" option in milli
-backoffMaxDelay: node-backoff ""maxDelay" option in milli

Relies on node-backoff for the backoff implementation. 

Sorry... no tests =/