Redirect root domain URLs for docs to default version...without hitting Django app

WIP to address subdomain redirect issue #619. This adds the nginx pieces to redirect, but doesn't yet touch the production nginx config deployed from fabric, or the build task update of the settings file. The settings file is currently located per-project in `/home/docs/sites/readthedocs.org/checkouts/readthedocs.org/user_builds/<project>/metadata.json`, and configures the default language and version:

```json
{"version": "not-latest", "language": "en"}
```

Changes:
* Add ReadTheDocs.pm for reading settings file as json
* Add tests for RTD.pm
* Update nginx salt config
* Adds to project save to add a static metadata output