Add a way to launch MapReduce jobs

This implements a way to launch MapReduce jobs. The job description is
forced to be application/json, and likewise, the response will be in
JSON too.

A dev-dependency on data.json was added to make the test case nicer, and
to showcase how to use the map/reduce functionality.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>