Adds prepend term.

This includes implementing it in JS, Python and Ruby drivers and
polyglot tests for the lot of them.
adds onFinished callback to each
changes test to use onFinished
adds some documentation
adds return false to each to stop iteration
fixes little problem with each return val
adds documentation for each callback return value
Add prepend to the docs.
Merge branch 'next' into merge_br
Merge branch 'next' into 443_onFinished_callback
Merge branch 'next' into sam_890