Please expose id_str, in_reply_to_id_str etc

Twitter IDs are now so large that most API clients - this one included - are failing to handle them properly as numbers. Twitter have thus provided a string version for each, with names ending in _str.

Try it - grab any recent tweet. Compare `tweet.id` with `tweet.attrs[:id_str]` -- they are different numbers.

We now need to use these _str versions to successfully reply to and retweet statuses, otherwise we are trying to reply to a different status (or one that might not exist -- this could be the root cause of #362).

Could you please expose the _str attributes out so that they are public and we can see them in the docs? (Unless there is a clever way of supporting very large numbers in the existing Integer `id`, `in_reply_to_id` etc. values. I am new to Ruby, so I may have missed something.)