Update Oasis.

- nested sandboxes will now default to the parent sandbox oasisURL.
Make conductorURL configurable.

```js
var conductor =
  new Conductor({
    conductorURL: '/path/to/conductor.js.html'
  });
```

Nested cards default to the conductorURL of their parent.

Fix #20.
Merge pull request #48 from tildeio/configurable-url

Make conductorURL configurable.