j(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
take note that we're using distinct here to mirror the existing behavior of fact-names.  It seems possible to me that the endpoint would be more convenient if it dropped the distinct and included certname in the response, but I may not understand the relevant use case. What I'm describing would also be a step closer to the existing behavior of fact-nodes, though that doesn't include type.