Added @GridFS

```php
/**
 *  @Persist
 *  @GridFs
 */
class Files
{
    /** @Id */
    public $path;

    /** @String @Required */
    public $metadata_1;

    /** @String @Required */
    public $metadata_2;

    /** @Stream */
    public $fs
}

$f = new Files;
$f->path = "/xxx";
$f->metadata_1 = "foo";
$f->metadata_2 = "bar";
$connection->file($f)->storeFile(__FILE__);

$file = $connection->getCollection('fs')->getById("/xxx");

echo "$file->metadata_2\n";

// @Stream can be used to read the file as if it was local
echo fread($file->fs, 1024);
```

Current limitations:

Files and their metadata are read only