Don't set list_id in GetListTweets when listID is zero

[tSU-RooT]

Twitter API lists/statuses requires 'list_id' or 'slug',

I tested GetListTweets with slug and other required parameters, like this below.

Sample Code:

api := anaconda.NewTwitterApi(Token, TokenSecret)
val := url.Values{}
val.Add("owner_screen_name", "foo")
val.Add("slug", "bar")

tweets, err := api.GetListTweets(0, true, val)
// An error occurs!
if err != nil {
    panic(err)
}

If listID is zero, ignore it and we can use the lists API.

[muesli]

@tSU-RooT: Similar to #157, I'd prefer a convenience method to get lists by slug. What do you think?