First production release

• Generated passphrase will now be printed to a file (cwd/passphrase.ghost) instead of to the screen.
• Add 99% test coverage.
• Support Python 3.
• Remove logger in favor of click's echo.
• Allow to choose backend using the CLI.
• Add export and load CLI commands and APIs to allow to export all keys in a stash to a file and then load them, potentially to a stash of a different backend.
• Add purge API and CLI command to delete all keys in a stash.
• Replace --value options with unlimited arguments in the form of key=value for the value field of a key.
• Add --no-decrypt flag to the CLI when running ghost get.
• Use SQLAlchemy PickleType for values in the SQLAlchemy Storage.
• Add -j flag to jsonify key list.
• Remove T from the timestamp in the modified_at and created_at fields of a key.