Create trakt_remove.py

As discussed here : https://github.com/Flexget/Flexget/pull/88, this new plugin can be used to remove a collected movie from a trakt.tv list (Watchlist or custom list). I'll use this to keep my Watchlist with movies I haven't collected yet, but you can also create a "to collect" custom list.

The use is very similar to trakt_acquired, it just need one more parameter : the list name.
The code is very close to trakt_acquired, I just changed the post_url and the json item to fit these requirements: http://trakt.tv/api-docs/lists-items-delete 

I tested and debuged it and everything is working great!