(PDB-711) Add factsets endpoint to V4 PuppetDB API

This pull request adds the /factsets endpoint specified in the
"Structured Facts Design and Plan" document circulated to Puppet-dev
on June 30.  The response is a list of maps, each describing the
facts, environment, and time of most recent fact submission for
an agent node.
Style, docstrings, and what candidates here for schematizing?