Compatibility with watson/validating package

When model is implementing ValidatingTrait from [watson/validating](https://github.com/dwightwatson/validating) package, Sentry isn't running `validate` method and thanks to this isn't throwing exceptions, but is returning true/false depending on whether the model is correct.

Now we can use something like this (Laravel):

```
$user = Sentry::register($credentials);

if (!$user->save()) {
    return Redirect::back()->withErrors($user->getErrors()); // fail
}
return Redirect::home(); //success
```

(Sorry for my bad english)