(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
Maybe describe the motivation as well. That is, its meant to be useful for autocompletion in GUI's, or for other purposes that require a global view of all possible fact-paths, such as an initial view for a GUI for browsing facts from a global perspective.

Of course people get creative, so it might be used for other reasons, but the API developer would probably like a hint behind its original purpose I figure.