Remove libuv, librustuv, gyp.

Remove libuv, librustuv, and gyp in preparation for the removal of libgreen.
Please put a link to the [runtime removal RFC](https://github.com/rust-lang/rfcs/pull/230) and explain that I/O support with libgreen is being removed.

Probably a good idea at the top of the crate documentation to explain the transitional situation, and that libgreen will eventually be removed entirely.