The spacing between each column. Can be : <br/>
 - empty: no gutter<br/>
 - a size in pixel (ex. <em>10px</em>) <br/>
 - a size in percentage (ex. <em>10%</em>) <br/>
 - a CSS selector (ex. <em>#column-size</em>) <br/>
 See the <a href="http://masonry.desandro.com/options.html#gutter">masonry doc</a> for more information.