Bring back hyper with proxy support

[inejge]

Reintegrate hyper support and make it selectable at runtime by setting the environment. Add proxy support to downloads handled by hyper (also activated by setting the environment.)

Fixes #509

[brson]

I retried just to see what happens.

[alexcrichton]

I believe the CI error means that Cargo.lock may not be up to date and the CI builders are trying to modify the readonly copy. Perhaps a cargo build locally and being sure to check in that updated Cargo.lock would work?

[inejge]

I believe the CI error means that Cargo.lock may not be up to date and the CI builders are trying to modify the readonly copy.

Exactly right. Travis builds are now passing, thanks and sorry for the noise.

[brson]

Thanks @inejge!