JobType Rework

This pull request is a rework of the base `JobType` class and associated modules.  The changes here are based on a thread on [discuss@pyfarm.net](https://groups.google.com/a/pyfarm.net/d/msg/discuss/er2e44kiPBE/JwVnOn43U-4J).
I like this idea quite a bit because you're right that given the architecture it would be difficult to get this working right.  That said, what about pushing the path maps when work is assigned in addition to when the argent starts/periodically? The thing I want to try and avoid is running with an out of date path map.