Small codechanges regarding default finder and find_elements

I created a small patch wich allows you to change the default finder from xpath to something different like css (in my case). 
The second change is about always returning an array in the find elements sub to be possible to use it in array context like  "foreach($self->driver->find_elements('#tab-panel button')) {", now it will always return an array (if nothing found an empty one).
I adjusted the POD and rerun all unit tests, so please take a look and make suggestions if you disagree with some changes.