Implemented Duration arithmetic operators +, -, +=, -=.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/10422

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1468308 13f79535-47bb-0310-9956-ffa450edef68
Changed Clock::advance() and Clock::update() to use Duration.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/10421

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1468309 13f79535-47bb-0310-9956-ffa450edef68
Fixed tests to use new Clock::advance() and Clock::update() functions
that take a Duration.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/10442

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1468310 13f79535-47bb-0310-9956-ffa450edef68