Fix for issue #91.

This issue appeared because ScnSocialAuth did event stopPropogation, and this approach had worked before ZfcUser was not changed to rise exceptions when event was stopped.

Previous in `ZfcUser/Authentication/Adapter/AdapterChain.php` :
```
public function prepareForAuthentication(Request $request) {
...
if ($result->stopped()) {
            if($result->last() instanceof Response) {
                return $result->last();
            } else {
                // throw new Exception('Auth event was stopped without a response.');
            }
        }
```
Now in `ZfcUser/Authentication/Adapter/AdapterChain.php` :
```
public function prepareForAuthentication(Request $request) {
...
if ($result->stopped()) {
            if($result->last() instanceof Response) {
                return $result->last();
            }

            throw new Exception\AuthenticationEventException(
                sprintf(
                    'Auth event was stopped without a response. Got "%s" instead',
                    is_object($result->last()) ? get_class($result->last()) : gettype($result->last())
                )
            );
        }
```