Protect imdb_id field from rottentomatoes_lookup

If the alternate_ids list in rottentomatoes_lookup's movie object does
not contains a valid imdb_id, the plugin assign it to None on target
entry, eventually clearing a real value.
Merge pull request #234 from tarzasai/protect_imdb_id

Protect imdb_id field from rottentomatoes_lookup
Add twitter feed plugin + eztv and shortened url rewriter