Fix Drupal Queue integration

This fixes the abovementioned issues for Drupal 7. Most importantly, job schedules need to be declared in a hook now, this is in analogy to how drupal's queue works. Compare this hook to hook_cron_queue_info().

/**
 * Declare job scheduling holding items that need to be run periodically.
 *
 * @return
 *   An associative array where the key is the queue name and the value is
 *   again an associative array. Possible keys are:
 *   - 'worker callback': The name of the function to call. It will be called
 *     at schedule time.
 *   - 'queue name': The name of the queue to use to queue this task. Must
 *     contain a valid queue name, declared by hook_cron_queue_info().
 *   If queue name is given, worker callback will be ignored.
 *
 * @see hook_cron_job_schedule_info_alter()
 * @see hook_cron_queue_info()
 * @see hook_cron_queue_info_alter()
 */
function hook_cron_job_schedule_info() {
  $queues['import'] = array(
    'worker callback' => 'my_import_routine',
  );
  $queues['clear caches'] = array(
    'queue name' => 'my_cache_clear_queue',
  );
  return $queues;
}

Improves the example in the hook documentation.

Further unify concepts of queue and scheduler.

- One object per scheduler identified by $name.
- Hence, move cron dispatching routine out of class into module.
- JobScheduler::get($name) instead of job_scheduler($name).

Also updated README.txt and job_scheduler.api.php accordingly.

- Fix a couple of typos.
- Remove semaphore. The scheduling routine being time based plus the recovery of stuck scheduled flags should make this unnecessary.

Fix: only reserves job when using the queue.

- Fix some typos
- Move exception that gets thrown when no scheduler info can be found to dispatch() - allows for scheduling items that don't have an info callback declared /yet/

Fixed further typos. This is very close now. Will need an update to Feeds.

This is tested now, will commit soon together with an according commit to Feeds.

Compatibility patch http://github.com/lxbarth/Feeds/commit/ff48db14abc57cc5801d92001263f7365...

This is committed now. Needs to be ported to Drupal 6.

#11: should be fairly easy to back port. I haven't started on it at all though.