Scheduler

For the first step in building precompute functionality into Jia, this pull request introduces a general purpose task scheduler. See [jia/scheduler/README](https://github.com/Locu/kronos/blob/scheduler/jia/scheduler/README.md) for a sample.
Haha yes, I would love an alternative suggestion. I originally named it `scheduler.py` but that caused some problems when I needed to do `from scheduler import ...` and it was importing from this file instead of the package. Maybe I just don't know what I'm doing with Python imports... but I didn't want to tackle that problem and just named it something dumb instead.