Add optional "source" parameter to update_status.

Note: source parameter only works with identi.ca API.
Twitter ignores this parameter.

Signed-off-by: Joshua Roesslein <jroesslein@gmail.com>

tweepy/api.py

@@ -104,7 +104,7 @@ def __init__(self, auth_handler=None,
         path = '/statuses/update.json',
         method = 'POST',
         parser = parse_status,
-        allowed_param = ['status', 'in_reply_to_status_id', 'lat', 'long'],
+        allowed_param = ['status', 'in_reply_to_status_id', 'lat', 'long', 'source'],
         require_auth = True
     )