Add GaufretteDetector

I've created a [Gaufrette](https://github.com/KnpLabs/Gaufrette) Adapter for determining whether or not a file exists.

Example:

```` php
<?php

use Gaufrette\Filesystem;
use Gaufrette\Adapter\Local as LocalAdapter;
use Emarref\Namer\Strategy\SuffixStrategy;
use Emarref\Namer\Detector\GaufretteDetector;
use Emarref\Namer\Namer;

$adapter = new LocalAdapter('/var/media');
$filesystem = new Filesystem($adapter); 

$strategy = new Strategy\SuffixStrategy();
$detector = new GaufretteDetector($adapter);
$namer    = new Emarref\Namer($strategy, $detector);
echo $namer->getName('Taken'); 
````
I'm not exactly sure on how best to write the tests for this though. Any suggestions?