ETag support

Adds [HTTP ETag](http://en.wikipedia.org/wiki/HTTP_ETag) support, proven solution to save a lot of network bandwidth. Usage:

```php
// Send request with no ETag check
$response = (
  new FacebookRequest(
    FacebookTestHelper::$testSession,
    'GET',
    '/104048449631599'
  ))->execute();

// Save ETag for future use
$etag = $response->getETag();

// Send request **with** ETag check (6th FacebookRequest constructor parameter)
$response = (
  new FacebookRequest(
    FacebookTestHelper::$testSession,
    'GET',
    '/104048449631599',
    null,
    null,
    $etag
  ))->execute();

if ($response->isETagHit()) {
  // Content didn't change
} else {
  // Get a new ETag and cache a new content
  $etag = $response->getETag();
}
```
Tests can be found in `tests/ETagTest.php` file.