Merge pull request #29 from guangmingyao/yao_dev

Yao dev
Merge pull request #31 from guangmingyao/yao_dev

changed some typos
Changed the date and one color change for the growth/decay lecture.
Merge pull request #47 from KellyBlack/black_dev

Change of date and one color change.
changed the date for friday
fixed the conflict
week3 monday note: add colors, changed a few notations
Merge pull request #32 from guangmingyao/yao_dev

Yao dev