Add import/export to CLI.

This commit adds two new commands:

    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.
Merge pull request #128 from benbjohnson/import-export

Add import/export to CLI.