(FACT-766) Add weight param to DirectoryLoader#new

Allows users of this class to specify a weight for the created facts,
without having to hack around the use of a constant.
(FACT-766) Fix DirectoryLoader default weight spec

This test never worked properly; inverting the `-` to a `+`, giving the
internally-loaded fact a weight *higher* than the external fact didn't
break the spec. Changing this to use `collection.add` rather than the
global `Facter.add` fixes the issue.
Merge pull request #824 from dgoodlad/directory-loader-configurable-weight

(FACT-766) Directory loader configurable weight
* (FACT-766) Add weight param to DirectoryLoader#new
* (FACT-766) Fix DirectoryLoader default weight spec