Run a directory full of tests

It's now possible to supply a directory to the JavaScript
test runner and it will run all the .js files in that
direcotry.
Tests for replicator use_checkpoints option
Add documentation for use_checkpoints replicator option
docs: various updates for release notes for 1.6.0

- closes COUCHDB-1795
- closes COUCHDB-1962
Document use_checkpoints and checkpoint_interval config options.