Version 0.1.15

* Improve performance of `BiLock` methods
* Implement `Clone` for `FutureResult`
* Forward `Stream` trait through `SinkMapErr`
* Add `stream::futures_ordered` next to `futures_unordered`
* Reimplement `Stream::buffered` on top of `stream::futures_ordered` (much more
  efficient at scale).
* Add a `with_notify` function for abstractions which previously required
  `UnparkEvent`.
* Add `get_ref`/`get_mut`/`into_inner` functions for stream take/skip methods
* Add a `Clone` implementation for `SharedItem` and `SharedError`
* Add a `mpsc::spawn` function to spawn a `Stream` into an `Executor`
* Add a `reunite` function for `BiLock` and the split stream/sink types to
  rejoin two halves and reclaim the original item.
* Add `stream::poll_fn` to behave similarly to `future::poll_fn`
* Add `Sink::with_flat_map` like `Iterator::flat_map`
* Bump the minimum Rust version to 1.13.0
* Expose `AtomicTask` in the public API for managing synchronization around task
  notifications.
* Unify the `Canceled` type of the `sync` and `unsync` modules.
* Deprecate the `boxed` methods. These methods have caused more confusion than
  they've solved historically, so it's recommended to use a local extension
  trait or a local helper instead of the trait-based methods.
* Deprecate the `Stream::merge` method as it's less ergonomic than `select`.
* Add `oneshot::Sender::is_canceled` to test if a oneshot is canceled off a
  task.
* Deprecates `UnboundedSender::send` in favor of a method named `unbounded_send`
  to avoid a conflict with `Sink::send`.
* Deprecate the `stream::iter` function in favor of an `stream::iter_ok` adaptor
  to avoid the need to deal with `Result` manually.
* Add an `inspect` function to the `Future` and `Stream` traits along the lines
  of `Iterator::inspect`