Command-line tips from blog post

Mentioned in #1, [this blog post](http://owenou.com/2012/01/13/ten-things-you-didnt-know-git-and-github-could-do.html) has some good tips that we should add:
- `git log -S'pattern'`
- `git log ..BRANCH`
- `git branch --contains`
- `git shortlog -sn`
