Add readonly client.

This PR is not complete, but it's a start.

I'd like to be able to create jobs over something like [juliangruber/multilevel](https://github.com/juliangruber/multilevel) where the context which is creating the jobs doesn't have access to the original Job instance or the 'worker'.

This PR allows you to only pass in the correct sublevel to start creating jobs:

```js
// client.js
var Queue = require('level-jobs/client');
// ...
var queue = Queue(db.sublevels['my-tasks'])
queue.push('my-work') // inserts job into db, should trigger `workerFn` on server
```

```js
// server.js
Jobs(db.sublevel('jobs'), workerFn)
```

**The problem is inserting the new job doesn't actually trigger the job to start**; the server end doesn't know a new job was added because we didn't go through its `push` method which triggers the jobs.

Would like your input on possible good way to solve this. `setInterval` polling? [dominictarr/level-live-stream](https://github.com/dominictarr/level-live-stream) from `q._work`?