Priority event hooks

Hi,
I have added prioritization for callbacks.
The sample: 
It will create two extensions the name_2 extension has higher priority so will always be called first. The default priority is 0. I have updated the readme.md accordingly.

```javasscript
$.nette.ext('name', {
			//hooks
		}, {priority: 100});
$.nette.ext('name_2', {
			//hooks
		}, {priority: 200});
```