Merge pull request #95 from jennybc/master

Finished updating citations of tidy data paper
expanded motivation for R lesson
fixed typo

changed its to it is
corrected a spelling error
minor typo fixes
Update 03-dates-as-data.md
Added to make a more complete bullet
edited a dash

There was a space missing between a dash and a word. I added a space.
added a "the" under sort 

"Bad values often sort to bottom or top of *the* column."
Changed contraction to two words, added a few spaces for formatting
small word change on line 29
Update 04-loop.md

Added a space between a ; and a word.
removed word

There was an extra "a" article in the first line.
Merge pull request #105 from agartland/patch-1

Added to make a more complete bullet
Merge pull request #108 from suparee/patch-1

removed word
Merge pull request #107 from sophieclayton/patch-1

small word change on line 29
Merge pull request #106 from anakarinarm/patch-1

Update 04-loop.md
Merge pull request #104 from jaclynsaunders/patch-1

added a "the" under sort
Merge pull request #102 from rcarns/patch-1

Minor edits
Merge pull request #101 from datacarpentry-test/patch-1

edited a dash