Start list comprehensions in part 4
Update prompt counts
Add dictionary example
Add extra credit for dictionary example
Remove output cells from dictionary explanation
Add sets explanation
corrected description of functions
fixed typo
Add membership and size checking for dicts
Merge pull request #4 from pythonsd/part-4

Part 4
Merge pull request #8 from pythonsd/fix_annotations

Fix annotations
Changed names of variables and some of the text
Part of function exercise.
Merge branch 'master' of github.com:pythonsd/intro-to-python
Improve wording and ordering of Part 3.
Boosted the loop section, added lists of lists, and sets.
Detailed review of part 3 greeting section.
Merge branch 'master' of github.com:pythonsd/intro-to-python
For fun, add an advanced str example to Part 3
Add age factor example to Part 3.