Deduplicate docs, move olds specs and prototypes to prototypes branch

Closes #723
Tidy display code
Cheapen index creation, speed up symbol column lookup

If columns groups come back, at least restrict storage to Int (!Real)
Fix stack overflows, etc., in stack and stackdf, of all names
Merge pull request #730 from JuliaStats/unifydocs

Bugfixes, remove old doc folder, move out old prototypes and specs
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`.