Add binding.gyp to build with recent versions of Node

On some systems, `eio.node` is building properly using `node-waf`.  However, elsewhere (running node-0.8.21/0.8.22), it's not.

[`node-gyp`](https://github.com/TooTallNate/node-gyp) uses `binding.gyp` to build on 0.8.x.  `#include <eio.h>` was removed, as `node-gyp` does not expose EIO headers.

(This does not work on 0.10.x, as EIO is completely unavailable, but it addresses short-term breakage.)