Kill a running action

Resolves #31

This adds support for killing a running action, and lays the framework for resuming jobs after a restart of trond, and for having a max runtime for jobs.

* Add an optional set of scripts which wrap the execution of each action.  This action runner is required to be able to kill running actions
* Adds the `action_runner` section to the config to enable and configure the optional action runner

(branched from https://github.com/dnephin/Tron/tree/136_scheduler_jitter)