Hyper backend

[yoshuawuyts]

In https://github.com/http-rs/surf/blob/8cf05e347404abbb091b80f809a5e1a354c6c735/src/http_client/hyper.rs we have a hyper 0.12 backend that unfortunately doesn't quite work. I fumbled something up during release, and never quite fixed it. But now it seems there may still be a few issues to resolve.

It'd be nice if we could port it back here, and restore it under the hyper-client flag. Note that this should be based on Hyper 0.12, and not any of the alphas as a lot of it still seems to be in flux. Thanks!

[goto-bus-stop]

hyper 0.13 is officially out now, so this should use it over 0.12 probably!

[calvinbrown085]

I assume this would be adding something similar to https://github.com/http-rs/http-client/blob/master/Cargo.toml#L18 here.

Something like
hyper_client= ["hyper-client]
?

I'm willing to help pick this one up if I could help get it across the finish line :)

[bspeice]

I think this can be closed now that #32 is merged?

[goto-bus-stop]

looks that way, thanks!