Use Cargo features to restore Windows compatibility

See this: http://crates.io/manifest.html#the-[features]-section

Using `glfw-rs` on windows:
```toml
[dependencies.glfw-rs]
git = "https://github.com/bjz/glfw-rs"
default-features = false
```

Building `glfw-rs` on Windows: `cargo build --no-default-feature`

This will prevent the `glfw-sys` dependency from being compiled.

On any other platform, it stays the same.

That was the behavior before the addition of `glfw-sys`.