misc updates

hi, i like what you have here and wanted to give it some love. :heart:

- i updated the db to be parametric so as to allow for any type of db, whether with levelup, level.js, multilevel, sublevels, etc.
- i made the library return a sync function rather than a new Backbone.Model so as not to enforce any one type of usage if someone wants multiple sync methods.
- i added collection read support, so fetching a collection with a sync function from a db will return all objects in that db. (sublevels should be handy for this)
- i also cleaned up the tests using chai expect.