(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
Come to think of it, isn't fact_paths already unique for path & type?

````
CREATE TABLE fact_paths
(
  id bigint NOT NULL DEFAULT nextval('fact_paths_id_seq'::regclass),
  value_type_id bigint NOT NULL,
  depth integer NOT NULL,
  name character varying(1024),
  path text NOT NULL,
  CONSTRAINT fact_paths_pkey PRIMARY KEY (id),
  CONSTRAINT fact_paths_value_type_id FOREIGN KEY (value_type_id)
      REFERENCES value_types (id) MATCH SIMPLE
      ON UPDATE RESTRICT ON DELETE RESTRICT,
  CONSTRAINT fact_paths_path_type_id_key UNIQUE (path, value_type_id)
)
````