Fixed db.batch(), improved tests, added readme note on db.batch() chained, updated to level / fixes #5

- `db.batch()` had multiple bugs and was totally non-functional, so I've fixed that and improved the tests to make sure it does work (it inserts the records into the db for ex).

- I've added a README note that we don't support the chained interface for `db.batch()`, but only the array one (I guess at the time you wrote this lib there was only the array interface). We can support this later if we want to though.

- I've updated to `level` last version instead of a really old `levelup` version (dev deps, package.json)

I've added some comments to the code.