Daemonizing server on unix platforms

Here is a draft implementation of daemonizing for Unix systems. Here is a rough idea of how it works:

```
/*
 * Daemonizing on unix
 * 
 * Uses the R event loop to trigger the libuv default loop. This is a similar mechanism as that used by Rhttpd.
 * It adds an event listener on the port where the TCP server was created by libuv. This triggers uv_run on the
 * default loop any time there is an event on the server port. It also adds an event listener to a file descriptor
 * exposed by the uv_default_loop to trigger uv_run whenever necessary. It uses the non-blocking version
 * of uv_run (UV_RUN_NOWAIT). 
 *
 */
```