Version 0.1.8

* Fix some race conditions in the `Shared` implementation
* Add `Stream::take_while`
* Fix an unwrap in `stream::futures_unordered`
* Generalize `Stream::for_each`
* Add `Stream::chain`
* Add `stream::repeat`
* Relax `&mut self` to `&self` in `UnboundedSender::send`