Add index support

Adds support for various operations on indices:

```js
db.index.list().then(console.log.bind(console));

db.index.get('dictionary').then(console.log.bind(console));

db.index.get('dictionary').then(function (dictionary) {
  return dictionary.set('foo', '#1:2');
});

db.index.get('dictionary').then(function (dictionary) {
  return dictionary.get('foo').then(console.log.bind(console));
});
```