Update plugin_transmission.py

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)
Let dump try converting field values to strings

Values in some fields can be objects, which may be able to produce
their own string representations. Give it a shot before outputting
'not printable'. Mostly for series_parser.
Remove broken imdb_rated plugin
Fix a bug in subtitles plugin
Make sure imdb_id field is being used over imdb_url
Cleanups
Find plugin handles filesystem encoding better.
refs #2295, #2065, #989, #1347
Make path encoding/decoding helpers a bit closer to python 3 ones
Fix --explain-sql
Add back --profile
Silence an ImportWarning from deluge plugin
Work around encoding bug in tmdb3. refs #2392
Make sure find plugin doesn't crash with invalid timestamps. fix #2434
New trakt.tv acquire/remove plugins

Movies, series and episodes are automatically recognized and
added/removed to/from the trakt watchlists, collections, seen and custom
lists.
Small fixes
Merge pull request #203 from tarzasai/trakt_submit

New trakt.tv add/remove plugins
Add no_entries_ok plugin to silence nuisance warning messages
Make pylint be quiet
Better sample config; bug fixes

Provide better sample configuration for cli
Catch additional render errors
Bug fix - use correct method of allowing aria2 to set gid
Change API URL