Updated analytics 2

This pull request includes two new rake tasks:

```
rake backup:analytics
```

To be run on the server to back up the analytics mongo database to S3 (akin to the `rake db` aka `rake backup:db` task).

And of course the analyzed CSV analytics export, which currently exports for all courses:

```
EXPORT_DIR=/path/to/backup rake analytics:export:courses:csv_analyzed
```