bundle expected libuv via git subtree

As evidenced by #13 it is surprising to people that libuv is fetched at ``make``-time. This also necessitates an Internet connection *and* GitHub being up in order to compile neovim. Both of these requirements are similarly surprising to experienced developers.

Since we mandate a particular version of libuv and are reticent to allow the use of system-supplied libraries, it makes sense to use ``git subtree`` to bundle that version directly into a ``third-party`` directory and modify the existing ``get-libuv.sh`` script to only compile that version. The use of ``git subtree`` also has the advantage that a) the precise commit of libuv being used is recorded in the repository history and b) we can move to a later version of libuv directly by appropriate ``git subtree`` incantation recorded in source history rather than fiddling with ``get-libuv.sh``.

Such an incantation might be:
```console
$ git subtree merge --prefix=third-party/libuv --squash https://github.com/joyent/libuv $TAG
```
where ``$TAG`` is replaced by the tag name one wishes to merge.

Use the ``--squash`` parameter to ``git subtree`` to avoid pulling in the entirety of libuv's history. This makes the difference in bandwidth required between bundling libuv and fetching a tarball directly negligible.

The version of libuv which has been merged corresponds to the "v0.11.19" tag which at merge time pointed to joyent/libuv@336a182.

An alternative solution would be to separate compilation from dependency fetching. Perhaps moving the fetching to the ``make cmake`` stage?

This PR does not completely address #13 since, unless one notices the "build" instructions in the README, the usual ``mkdir build && cd build && cmake .. && make`` recipe for compilation will fail unexpectedly which is likely to lead to lots of #13 dupes being opened. Should this PR be merged, it may be prudent to modify the CMakeLists.txt file to compile the provided libuv either by "CMake-izing" libuv or calling ``compile-libuv.sh`` from CMake. This would allow compilation using the standard CMake build procedure. If nothing else, this would make packaging neovim a lot easier.