PostRetweet function implements retweet API

Added a PostRetweet function which implements the retweet API.

https://dev.twitter.com/docs/api/1/post/statuses/retweet/%3Aid

Also works with Twitter API v1.1.
Merge pull request #46 from sebastianw/retweet

PostRetweet function implements retweet API
Merge branch 'master' into hashbang