(#66) Stop puppet internals leaking into facts.yaml

Prior to this patch we'd take everything in scope that didn't have a dynamic
name.  This patch extends it to reject anything with a value that's not of class String.

This may bite once structured facts becomes a thing, depending on how they look.

Closes #66
Merge pull request #67 from richardc/bug/master/66

(#66) Stop puppet internals leaking into facts.yaml