Create trakt_remove.py

I created trakt_remove.py as discussed here : https://github.com/Flexget/Flexget/pull/88
I've debugged and tested it and it's working fine for movies, I didn't developped it for TV Shows yet as it doesn't seems really necessary.
With this plugin you can ask trakt.tv to remove collected movies from a list (Watchlist or custom).
The code is practically entirely based on trakt_acquired, I just changed the post_url and change the JSON item to respect trakt.tv API : http://trakt.tv/api-docs/lists-items-delete

I hope you'll find this usefull!