Revert "Revert "Merge pull request #891 from lukecampbell/validate_unit""

This reverts commit 271ed8c5868450beaf21f837deb735d412d769e7.
Revert "Revert "Adds udunitspy to setup.py""

This reverts commit 6036fe9d867e8b246ecfd38f2e6af22c6b9550d8.
Updated README