Updates to reshape functions and updated documentation

I added a few more methods for `stack` and `unstack` and added documentation. 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.


Not super comfortable with this methods that picks what to stack based on eltype. I wonder what others think.