Allow an existing express application to "mount" many multilevel-http instances

1. Just not assuming you have to create "app": can we accept an app parameter and eventually initialize it if not passed in?

2. Added meta.base: to allow an existing application to attach multilevel-http routes under a base path

3. Returning db, because if you are using multiple instances of multilevel-http on the same app, you may need to store it somewhere else instead of app.db.

Take a look at: https://github.com/aaaristo/dull/blob/master/server/bucket.js... Is a "dumb riak clone" that uses multilevel-http for buckets.
+1 on this