For now this only adds convenience wrappers around Guzzle to somewhat
emulate Diactoros (albeit with a different API). Code and tests will be
adjusted in due course.
The plan had originally been to use Bldr, but Bldr's features were not particularly compelling, and it has not been maintained recently. By contrast Robo has very useful features while still meeting my needs.
In addition to migrating the build process, the current Robo file also runs unit tests, with or without coverage reporting.
Fixes#116; improves #113