DOC :Cheatsheet update.

[Dr-Irv]

Update to cheatsheet, from suggestions from a colleague.

1. Fix bug in example for outer join
2. Add head() and tail() and use space for nsmallest()
3. Formatting of logic table.

[Dr-Irv]

@jreback fixed a few issues in the CheatSheet, so this version (the pptx and pdf) can just be copied over.

[jreback]

@Dr-Irv thanks. I merged. In the future do this.

git checkout master
git pull
git checkout -b <newbranch>

then you will have a new branch that starts with master