Worker wrapper package

This change defines the package that will be used by the worker wrapper
program. It implements the interface defines in the standalone interface
spec for the wrapper process.

It tests the library with a bunch of simple scripts that exercise each
of the components of the interface.

This relies on a couple mock changes in another PR which is why
the tests are failing.
I might not refer to the cmd to run as a "worker process", here and below, since we don't want to think of it as being specific to Gearman or a Gearman worker. Maybe just "wrapped process"? Something like `WorkerConfig defines the configuration for the wrapped process`