Added week 6 (Actors part 2)

I added the content for week 6, and reorganized a bit the Actors section
Merge pull request #18 from lpoulain/patch-7

Added week 6 (Actors part 2)
Minor updates on Actor

- Edited some sections to make sure that the code gets properly displayed.
- Changed a few things about the postRestart, postStop(), etc. (these are methods and not messages)
Merge pull request #19 from lpoulain/patch-8

Minor updates on Actor
Add link to Scala Cheat Sheet
Merge pull request #20 from longhua/patch-1

Add link to Scala Cheat Sheet
Add information regarding how to log actors.
Merge pull request #21 from JobaerChowdhury/patch-1

Add information regarding how to log actors.
Week 7
Merge pull request #22 from lpoulain/patch-9

Week 7
Update ReactiveCheatSheet.md

Changes to Promise: Fix nomad to monad, and description/example for failure case.
Merge pull request #23 from deeptibhatia/patch-1

Update ReactiveCheatSheet.md
typo (Evenr -> Event)

fixed a small typo
Merge pull request #24 from hochgi/patch-1

typo (Evenr -> Event)