Add import/export to CLI.

This commit adds two new commands:

```sh
$ bolt import --input INPUT PATH
$ bolt export PATH
```

This exports the database in a simple, nested, key/value JSON document. Each node in the document has a "key", a "value", and an optional "type". The key and value fields are both base64 encoded.

---

This pull request also includes fixes to the CLI tests and includes the CLI tests in `make test`.