Backup process to the Internet Archive

This adds a `backup` script to the root of the repository, to back up reports and bulk data to the Internet Archive. 

It's mildly intended to be generalizable (e.g. a backup to one's own S3 account), but not much -- it's hooked pretty tightly into `scripts/backup/ia.py`.

More details about how the Internet Archive's uploading system works can be found in #63. #63 won't be complete until the collection is fully uploaded to IA, and syncing on a regular basis. This PR covers the scripts that will be used in these processes, and that have been used to update what's there in the collection right now.

### Backing up individual reports

The primary use is like:

```bash
./backup [--ig] [--year] [--report_id] [--force] [--meta]
```

If you want to specify a specific `--report_id`, you have to specify an `--ig` and a `--year` too. By default, the backup script will mark archived reports with a little `ia.done` file in that report's directory, and not upload reports that already have one. The `--force` flag will override this behavior and always upload the specified reports. The `--meta` tag will *only* upload a report's `.json` data and not the report itself, which is really only useful for testing.

So a cronjob that wanted to keep the report archive in sync with IA could just run `./backup` every X hours or days. 

Reports will be uploaded to the `usinspectorsgeneral` collection at https://archive.org/details/usinspectorsgeneral, that the Internet Archive created for the project after we uploaded ~90-100 reports using this script. The reports will be submitted to the Archive's "derivation queue", which should provide each PDF-based report with a pleasant little report reading interface, [like this one](https://archive.org/details/us-inspectors-general.treasury-2014-OIG-14-023).

### Backing up a giant bulk data file

An alternate use is:

```bash
./backup --bulk=us-inspectors-general.bulk.zip
```

If given a `--bulk` flag with a path to a zip file, this will upload that file directly to the Archive, at
https://archive.org/details/us-inspectors-general.bulk. This item is marked as part of the `usinspectorsgeneral` collection, and is the permanent link I'm giving people (and linking to from the collection's description) as a quick way to download all ~40GB of the reports we have so far.

Using the bulk data backup means creating a `.zip` file of everything, excluding the `.done` files:

```bash
cd /path/to/inspectors-general/data
zip -r ../us-inspectors-general.bulk.zip * -x "*.done"
```

Then uploading that zip file to the Internet Archive with:

```bash
cd /path/to/inspectors-general
./backup --bulk=us-inspectors-general.bulk.zip
```

This would also be suitable to do via cron, but maybe monthly or weekly. It's an expensive operation for everyone.

### Next steps

The next step is a full re-download of reports. @divergentdave has done yeoman's work in detecting and removing duplicate IDs, and integrating duplicate ID checking into the main report fetching process. @spulec did similarly rigorous work going back and adding report type detection to all the scrapers. These efforts won't apply to the full archive I have on my server, or that Sunlight has on theirs, unless we re-download everything. It'd be more efficient to migrate the collection in-place, but that's not really feasible.

I've initiated that re-download on my servers. Once that's done, I'll do a full backup to the Internet Archive, using the methods above.

Finally, once those are done, I'll instrument a couple of cronjobs that will keep things in sync automatically. Once that's done, #63 will be fixed, and we'll have a pretty nice system on our hands.