fast_image_size() method accepts Buffer

image-size library accepts a Buffer as an argument. fast-image-size could do same thing with this patch.

Test code:
```js
var request = require("request");
var fast_image_size = require("./");
var url = "https://www.google.co.jp/images/srpr/logo11w.png";
var opt = { encoding: null };
request(url, opt, function(err, res, buffer){
  if (err) return console.error("[ERROR]", err);
  var img = fast_image_size(buffer);
  console.warn(img);
});
```

Thank you for contributing the simple and lightweight library.