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.

Default just means that it's used unless specified otherwise. It would probably break Servo if it wasn't the default.