Issue #6 - ReadDirPlus doesn't work on NFSv4 mounts

This adds ReadDirPlus back in for NFSv3 mounts, but has an exception
similar to the code used in nfsiostat. I had to expose another method
in nfsiostat.py has_readdirplus() since that was internal only data before.

~tommy
Merge pull request #11 from TJM/issue-6

Issue #6 - ReadDirPlus doesn't work on NFSv4 mounts