Replace Task.swift with ReactiveTask framework

See [Carthage/ReactiveTask](https://github.com/Carthage/ReactiveTask). Resolves #164.
I don't know the answer to the former. Maybe we could try installing the Carthage package on Travis, and verify that it runs without any errors?

This file _can_ be synthesized using `pkgbuild`, but we added our own customization on top of that. I don't know that there's any way to update it without clobbering our changes.