(PDB-718) Fact path endpoint

This implements a "fact-paths" endpoint that returns list of fact paths/types
according to the specification here:
https://docs.google.com/a/puppetlabs.com/document/d/1vWE6tBmsPmMIYKQWV8Pwes_NIR9lEMTYjUH8zJ_AlMI/edit?pli=1