Created custom exceptions

Hi,

I have created a pull request to throw custom exceptions so that code like the following is possible:

```
  try {
    $resp = $client->event_search($search_params);
  }
  catch (EventbriteException $e) {
    // Deal with API errors.
  }
 catch (Exception $e) {
    // Deal with generic PHP or connectivity errors here.
 }
```

This makes it possible to deal with errors returned from the Eventbrite API differently to those thrown by other code.

I have put the custom exceptions in the main Eventbrite.php file as it is all currently self-contained there, so let me know if you think they should be in separate file(s) to be required.