This patch relicenses the Tokio project exclusively under the MIT
license. Before this, the project was dual licensed under MIT and Apache
2. As such, switching to only MIT is permitted.
Fixes#202
This allows libraries that require access to reactor related types to
depend on this crate without having to depend on the entirety of Tokio.
For example, libraries that implement their custom I/O resource will
need to access `Registration` or `PollEvented`.
