fix the function name in lesson14 exerciseReturnFunctions lab.
fix slide spacing in lesson 14
Prepare lesson 15 and 16 labs.
fix the instruction problem on cookjie monster.
prepare lesson 16.
typo in lesson 16 groceryList
Prepare lesson 17 and 18.
merge upstream.
Merge remote-tracking branch 'upstream/gh-pages' into gh-pages
final presentation