gitio outputs only the needed short url

Instead of outputting the headers of the request we could just extract the needed Location header.

I have expanded the cURL flags to their long form for readability (short-form command-line arguments should be used only on the command line).

---

BTW, we could use the `http://git.io/create endpoint directly which returns the slug in the body of the response.
However we would not be able to provide a desired slug then.