Sandboxes can open windows.

The open window will inherit the sandbox restrictions.
You need to set `sandbox.popups` to true when creating the sandbox.

```js
  var sandbox = oasis.createSandbox({
    url: 'http://example.com/profile_viewer.html',
    capabilities: [ 'account' ],
    sandbox: {
      popups: true
    }
  });
```

Firefox support for `allow-popups` begins with the version 27.
See https://bugzilla.mozilla.org/show_bug.cgi?id=766282
Its implementation differs from webkit or internet explorer: it raises
an exception when failing to open a popup if the `allow-popups` flag is
missing.

[Fixes #4]