Update to tokio 0.2(alpha)

[Will-W]

Remove dependency on futures crate as std::future now has everything we need
Bump tokio dependencies
Port lib.rs and example

[berkowski]

Will, thanks for your PR. Between you and @12101111 I think the path forward is pretty clear. I've made a new branch (tokio-0.2.0-alpha.1) and intend to push a release to crates.io soon.

I think #19 will be the possible merge candidate, if only because it arrived first. However both of your PR's helped me sort out the new async stuff and I'll be crediting you both in the change log regardless of how the functionality gets in.

[Will-W]

No worries, whatever works best for the project.

[berkowski]

Thanks, Will. Pushed version 4.3.3-tokio-0.2.0-alpha.2 (what a mouthful) to crates.io