feat(database): implement index/query API

Implements:
- db.addIndex(name, {map: .., reduce: ..}, callback)
- db.removeIndex(name, callback)
- db.query(index, options, callback)

Implementation based on discussions:
https://github.com/hoodiehq/hoodie-plugins-api/issues/3
https://github.com/hoodiehq/hoodie.js/issues/70#issuecomment-20506841

Closes #3