Fix issue #17: Jenkins.app definitely prevents sleep while building

Earlier Jenkins.app set up a launchd job that prevented sleep for 60
seconds, then checked if Jenkins is idle before preventing sleep for
another 60 seconds.

Apparently it is possible for 10.8 system to fall asleep immediately
when a power assertion goes away.

Now Jenkins.app prevents sleep for 24 hours. The prevention is stopped
immediately when Jenkins becomes idle.

I guess it is still possible for 10.8 to sleep before Jenkins.app
polls Jenkins state.