v3 minimal

This PR implements a minimal set of v3 endpoints that are compatible with open source Puppet 3.6 (see #1).  This does not add any database fields, and thus is simultaneously compatible with v1 endpoints and existing databases (at the cost efficiency).  I will wait till 1.0 to add backward-incompatible database fields (e.g., store `metadata.json`, `file_md5`, update/creation dates, moving tags to releases, etc.).