Remove `sandbox.jackknife`.  It belongs in `sampling`.

I still like the premise behind `toolz.sandbox`.
Remove `jackknife` from docs too.
Clean up doc api: pluck, curry, do, identity, juxt, get_in, keyfilter, valfilter

Also removed `side_effects` from docs.
`memoize` was listed twice in `functoolz.__all__`.
Merge pull request #158 from eriknw/remove_jackknife

Remove `sandbox.jackknife`.  It belongs in `sampling`.