Implemented horizontal layout option...

It's been requested, +1'd, and now implemented! :D

Packery will default layoutMode to 'vertical'; however, you can now pass in layoutMode: 'horizontal' to acheive a horizontal layout. I've not tested it extensively, but everything else seems to work as before.

I've also copied/modified the add/remove example to work with the horizontal layout to demonstrate it's use and functionality.