Using percentage instead of pixels for columns width

Hi,

I really like the idea of Greeed for simple Pinterest-like grids (I'm sick of absolute positioning used by masonry scripts).  

But what about using percentages instead of pixels for columns width? It avoid calculating pixel widths (and the `pixelsLeft` part), and is more "responsive friendly".  

Actually, it's even possible to completely ditch the `width` by using `display: table;` for the grid and `display: table-cell;` for the columns but it's starting to be kinda hacky (especially when using inline styling)...

BTW, I also added the possibility to change the type of generated elements, to be more markup agnostic.