Make it build on Windows

[Meralis40]

This pull request permit to build libuv-sys on Windows with MSVC toolchains.

[ircnelson]

@Meralis40 you know how to fix libuv windows linking?: https://gist.github.com/ircnelson/b6bc4c1eefa0291fc7998e370c6402ad

Build works fine, but when I execute, it fails..

Installed:
[x] vs2015
[x] vs2017
[x] vsbuild tools c++
[x] latest version rust

cargo new libuv-sample --bin

Cargo.toml

[dependencies]
libuv-sys = { git = "https://github.com/Meralis40/libuv-rs.git" }

main.rs

extern crate libuv_sys;
fn main() {
    println!("Hello, world!");
}

[Meralis40]

I've done some modifications to build.rs.
You also need Python 2 in PATH before building libuv-sys (because libuv use gyp).

I've tested it with vs2015, stable & nightly.