(pouchdb/express-pouchdb#95) - add tests for longpoll changes

These verify that the http server's
_changes?feed=longpoll works as expected.

The tests pass before and after my fix for
express-pouchdb (pouchdb/express-pouchdb#96),
but the main difference is that you'll
notice a lot less being spewed out in the logs,
since it's actually longpolling.
So this test writes a doc, starts a live changes feed that skips that doc, then after 2 seconds it writes 3 documents then resolves when it sees the first change?

This is a very racy test, the live changes feed needs to be cancelled and we need to be sure the entire bulkdocs operation is complete before finishing the test