Merge pull request #1 from datacarpentry/master

Get changes from upstream into Jenny's fork
Tidy data lesson response to moving data to top-level data directory
More loose ends re: tidy data lesson
Restore a TODO indicating a link is needed

Tidy data lesson needs a link to a lesson about get data out of, e.g., Excel and into a delimited file
Merge pull request #72 from jennybc/master

Updating tidy-data lesson in response to moving the data to top-level data directory