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 was relying on gearman-go setting the data field here https://github.com/Clever/gearman-go/blob/master/worker/worker.go#L325. It gets the data from the return value of the Process function.

Let me know if that's not the right way to do it.