Added support for retweets

Retweets can be called by passing a tweets 'id_str' as a string to T.post(). New function, checkForRetweet adds the 'id_str' to the path itself per API 1.1 as opposed to optional params after the '?'
Input for retweet now an object. Added support for params on retweets, like trim_user.
Update readme with retweet example
Merge pull request #47 from mhkeller/master

Add support for retweets