Update plugin_transmission.py (base64encode & cli.add_torrent)

As of transmissionrpc-0.11-py2.7.egg/client.py
add() & add_uri() are deprecated.

Althought, I was getting transmissionError (Query failed with result:...) when trying to launch a new torrent. So I went in the rpc/client code and used the same syntax they use to base64 the data and everything'sback to normal working order ^^

Althought I suspect that line 339 (for id in r.keys(): ) won't work with the return from cli.add_torrent(), I'm not equipped to deal with that (don't know when it occurs)