add chmod function

You can now choose permission mode of your cache files and folders.

### use example ###

```php
$cache = new \Gregwar\Cache\Cache;
$cache->setCacheDirectory($cache_dir);
$old = umask(0);
$cache->chmod(0775,0664);

// get cache or convert
$img_out_path = $cache->getOrCreateFile($img_out,
    array(
        'younger-than' => $img_src_path
    ),
    function($cached_file){
        // resize image
        global $img_out_size, $img_src_path;
        $command = "convert -filter Lanczos -background none -resize "
            . escapeshellarg($img_out_size) ." "
            . escapeshellarg($img_src_path) . " " 
            . escapeshellarg($cached_file);
        return exec($command);
    });
umask($old);
```

### result cache content ###
```bash
16:41:06 [jclement@baker11:/srv/www/.handlers/flyresizr/.cache] cache(+3/-0) ± tree -pgu
.
`-- [drwxrwxr-x apache   apache  ]  local.sites
    `-- [drwxrwxr-x apache   apache  ]  l
        `-- [drwxrwxr-x apache   apache  ]  i
            `-- [drwxrwxr-x apache   apache  ]  n
                `-- [drwxrwxr-x apache   apache  ]  k
                    `-- [drwxrwxr-x apache   apache  ]  -
                        `-- [-rw-rw-r-- apache   apache  ]  link-200x.png
```