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>/settings.json`, and only configures the branch name so far:

```json
{"branch": "not-latest"}
```

These settings will be updated with the default branch name on the project build task.

Changes:
* Add ReadTheDocs.pm for reading settings file as json
* Add tests for RTD.pm
* Update nginx salt config