support duration format for nagios settings

In order to unify the usage of monitored_cron and avoid timing related
errors, support a single time format for it.
Add inline docs for function
remove support for days

As it is the goal of ths pull request to unify the format for all,
remove support for days, as periodicnoise doesn't support this unit.

Only compatibility problem is now "µs", but my Ruby foo is not enough to
support this in Ruby 1.8.7+, 1.9.x and 2.x with the same code.