Fixed bug in _findBackgrounds() which caused to ignore elements with dat...

...a-stellar-background if you pass them directly to stellar() function. Also replaced check for data property via "is([data-prop])" with "data('prop')".

1) $stuff.add() doesn't change $stuff, so $backgroundElements remains unchanged
http://api.jquery.com/add/ 

2) I think check with "data('something')" is more appropriate than "is([data-something])"

Thank you for such a nice library! :+1: