Added priority support to event listeners.

Added support for setting priorities to event listeners. This way, if a pattern matches on multiple listeners (or multiple listeners to server events) are executed based on their priority.

This also gives some more use to tell the dispatcher to stop all propagation of the event ($event->stopPropagation();).

This change is fully backwards compatible.

Things changed:
- Priority support
- Code documentation updates
- Readme updated to match changes