Merge pull request #2 from datacarpentry/master

Get changes from upstream into Jenny's fork
cite tidy data paper (no longer a preprint)

also work around a dplyr::left_join() bug that does not preserve factor levels (only used to make data untidy, i.e. no learner-facing joins are done)
cite tidy data paper (no longer a preprint)

also work around a dplyr::left_join() bug that does not preserve factor levels (only used to make data untidy, i.e. no learner-facing joins are done)
Merge branch 'master' of https://github.com/jennybc/datacarpentry

# Via Jennifer (Jenny) Bryan (1) and jennybc (1)
* 'master' of https://github.com/jennybc/datacarpentry:
  cite tidy data paper (no longer a preprint)
Merge pull request #95 from jennybc/master

Finished updating citations of tidy data paper