Update plugin_transmission.py (base64encode & cli.add_torrent & cli.change_torrent)

Finished verification of pull request #173

Updated the base encoding to use same behavior as transmissionrpc https://bitbucket.org/blueluna/transmissionrpc/src/c5237f6ed9e2c9ad5dccf7387bf0b6cc95785152/transmissionrpc/client.py?at=default#cl-704

Updated to use cli.change_torrent and cli.add_torrent instead of deprecated functions.