Adding beforeEnqueue event on Resque enqueue.

This so that we can stop jobs from queuing up via Event Hooks, so that we can do something like this to create a unique queue:

```php
class ResqueUniquePlugin {
  public function beforeEnqueue($class, array $arguments, $queue) {
    if (Resque::redis()->exists($arguments['unique_key'])) {
      return FALSE;
    }
    Resque::redis()->set($arguments['unique_key'], '1');
    return TRUE;
  }

  public function afterPerform(Resque_Job $job) {
    if (Resque::redis()->exists($job->payload['args'][0]['unique_key'])) {
      Resque::redis()->del($job->payload['args'][0]['unique_key']);
    }
  }

  public function onFailure($exception, Resque_Job $job) {
    if (Resque::redis()->exists($job->payload['args'][0]['unique_key'])) {
      Resque::redis()->del($job->payload['args'][0]['unique_key']);
    }
  }
}
```
Is this intentional? Setting the variable to `true` then overriding it?