Fix add policy

[barrh]

This patch tries to fix a issue #176, among others. Refactored add_policy, and added a few other fixes that I use that patch with.

Some insight: Originally this PR was part of another PR that wasn't merged. I broke it even further, and the original commit was divided here into two. (so either can be dropped before merged) Bringing them together is the logical thing to do, because they were tested together. However, I almost didn't test it with the current distiller master, other than the most elementary scenario. (e.g. thinning was not tested)

Edit: I've noticed one side-effect, the scheduling arguments specified in the YAML (start, end, and frequency) are now optional.