using streams2 for db.createReadStream()

This patch uses [readable-stream](https://npmjs.org/package/readable-stream)`.Readable` for the `db.createReadStream()` api to implement the readable half of [issue #106](https://github.com/rvagg/node-levelup/issues/106). Only a few minor tweaks to the existing test suite were necessary to work around a few streams2isms.

This patch also fixes the issue where calling .pause() will let one more element through before pausing and I was running into some issues with piping to multiple destinations that this patch also seems to have fixed.