added anon user & group mapping

Added anonuser and anongroup attributes to LWRP.  These attributes take the actual user and group names instead of UID/GID.  It will add the anonuid and anongid options to the export accordingly.

My motivation for doing this was rather simple.  I have a recipe that needed to create a user and then export a file system with 'all_squash'.  It failed during resource compilation because the user had not been added yet.  Performing the lookup on the uid/gid inside of the provider allowed it to happen after the user was created.