Upgrade libuv to the current master + our patches

There were two main differences with the old libuv and the master version:

1. The uv_last_error function is now gone. The error code returned by each
   function is the "last error" so now a UvError is just a wrapper around a
   c_int.
2. The repo no longer includes a makefile, and the build system has change.
   According to the build directions on joyent/libuv, this now downloads a `gyp`
   program into the `libuv/build` directory and builds using that. This
   shouldn't add any dependences on autotools or anything like that.

Closes #8407
Closes #6567
Closes #6315
Implement process bindings to libuv

Closes #6436