Make libuv a git submodule

[brson]

Now that we have an LLVM git submodule and have some experience building libuv, I think the path is clear to turn libuv into a submodule.

For this conversion I'd like to pull the libuv source out of the runtime directory, and put it in src/libuv. We'll probably have to include gyp in our source tree (unless we want to host a git mirror and make that a submodule as well).

[graydon]

Not sure why this reopened. Libuv is a submodule presently. Closing.