first draft for Timers

this is a first draft, it just mandates that packages that are willing to provide a native systemd timer *must* also provide a equivalent crontab to retain SysV init _et al_ compatibility. 

It also provide a simple way to avoid that both are running at the same time.

It does'nt try to overide point 9.5 of the Debian Policy; just suplement it.

@joshtriplett