Add monthly scheduling

Currently Scheduler has functions for recurring Jobs by minute, hour, day and week. The next logical extension would be by month (if requested enough by second or year might also be valid extensions). Difficulties might occur if we allow wider combinatorics of the smaller time intervals. Additionally a monthly function calls for additional options like a step variable to allow scheduling of "every second and fourth Friday of the month" and similar requests.

The API definition is open for discussion. Some ideas what a "complete" API might be able to do:

Possible API requirements

• "every second and fourth Friday of the month"
• "The last two Mondays of a month"
• "The 5th and 11th of a month at 23:52:33"
• "Every day between the 5th and the following friday of a month at 23:52:33"

Note that some of the requirements could also be fulfilled, if a monthly Job were to create new Jobs of a different kind in the Scheduler.