Add clear-selected attribute to clear all the selected items

[arielcr]

Hi @lukemurray

I finally found a way to clear all the tree, I don't know if it's the fanciest way of doing it, but it works.

I add a clear-selected attribute to the directive that basically what it does it that is clears and refresh all the tree when the variable that is passed to the clear-selected attribute is changed.

It works pretty nice for me. I also add some documentation of the attribute to the demo page, just for a quick reference on how to do it.
I add a clear-selected attribute to the directive that basically what it does it that is clears and refresh all the tree when the variable that is passed to the clear-selected attribute is changed.

Tell me what you think, I hope you can merge it into your branch!

Cheers!