Add ability to unmount nfs shares

[myii]

• Complements mount.mounted by using mount.unmounted
• Since device parameter causes warnings on older minion versions, included a test to determine whether the parameter should be used
• Other changes include:
  • Update pillar.example to show how unmount is to be used
  • Update README to include nfs.mount and nfs.unmount
  • Add standard manual edit warning to managed exports file