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.
i had a similar thought---we use it in Metis and Kronos, so no big deal to add the dependency here.

doesn't have to block the pull request---add if you'd like :)