ETags support

This PR adds ETag support. Example of usage:

```php
$facebook = ...
$query = '/zuck';

// Get ETag for a query
$result = $facebook->api($query);
$etag = $facebook->getReceivedETag();

// Check if something has changed
$facebook->setETag($etag);
$facebook->api($query);
if ($facebook->isETagHit())
{
    // ETag has not changed
}
else
{
    // ETag has changed, save new:
    $new_etag = $facebook->getReceivedETag();
}
```

PS. Test case added in tests/tests.php (testETags).