Add a tail operation (with some tests)

Fixes #22.
Make delete use dynamic row indexes.

Fixes #59.
Add docs page for tail operation
Merge pull request #60 from okfn/59-delete-indexes

Make delete use dynamic row indexes.
Merge pull request #58 from andylolz/22-tail-op

Add a tail operation (with some tests)
Remove horrible errors in outcsv

Not sure what was going on here. This is better now. csv().out.array()
expects an array of arrays.
Merge branch 'master' into 56-default-renderer

Conflicts:
	lib/operators.js