DOC :Cheatsheet update. 
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.

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