This adds documentation and a few more methods for `stack` and
`unstack`. The main changes are:

* Added defaults for stack and unstack with fewer arguments.

* Added a method for unstack, so an id column isn't needed.

* Removed pivottable. It wasn't documented, and there are better ways to
  aggregate.

* A few little bug fixes.
Merge pull request #734 from tshort/master

This adds documentation and a few more methods for `stack` and `unstack`.