Fix sidebars on 2.1.

This change adds some config keys lifted from the 2.0 `_config.yml` file.

Without these keys, the sidebar for each blog post says

```
Included file 'sidebars/' not found in _includes directory
```

This appears to be the same bug reported by https://twitter.com/digitadventures/status/295337631804448768

Please consider merging this patch for the benefit of other 2.1 bleeding edge testers :)

Thank you very, very much for Octopress!