Add priority to SearchStrategies

And tadaaa, here it is! Fixes #56 :)
Wow, I didn't know this issue. Priority sorting is also done in the [Symfony EventDispatcher component](https://github.com/symfony/symfony/blob/master/src/Symfony/Component/EventDispatcher/EventDispatcher.php#L181). They rather use `krsort` so they don't come into this.

Could we use krsort?