Migrate libuv into a submodule

[erickt]

This patch turns libuv into a submodule. This commit needs some work done ahead of time in order to work properly. Graydon needs to use github's fork on https://github.com/joyent/libuv and then needs to incorporate rust's build system customizations of libuv by doing:

git clone https://github.com/graydon/libuv.git
cd libuv
git remote add git://github.com/erickt/libuv.git
git fetch erickt
git checkout -b rust remotes/erickt/rust
git push origin rust

Once this is done this commit can be pulled in.

[brson]

It would probably be very difficult to merge this by now, and I don't think we have the will to commit to this strategy at the moment, so I'm going to go ahead and close the pull request. We can revisit in the future.