Consistently use nanoTime in Process.duration

The first duration emitted should always be near zero.

Considered adding further properties to check later values,
but that is slow and never perfectly sure to be accurate on all
machines and all executions.
duration test in ProcessSpec
Merge branch 'master' into duration
Merge pull request #52 from jessitron/duration

Process.duration compares millis to nanos
Merge branch 'master' of github.com:scalaz/scalaz-stream into topic/actor-wye