Sketch of the first hour

Initial sketch of what gets shown.
Create blobs-trees-commits.md

First level.
Delete blobs-trees-commits.md

Needs to be in a subdirectory.
Create screencast.md

Sketch of the second hour.
A complete second-hour sketch
Update screencast.md

Add a review section.
Merge pull request #1 from jsh/blobs-trees-commits

Blobs trees commits
Merge pull request #2 from jsh/content-addressable

Content addressable