Do not layout items that are isHidden

Per desandro/masonry#496, items that are isHidden should not need to be laid out. Consider changing `_getItemsForLayout` to

``` js
Masonry.prototype._getItemsForLayout = function( items ) {
var layoutItems = [];
for ( var i=0, len = items.length; i < len; i++ ) {
var item = items[i];
if ( !item.isIgnored && !item.isHidden ) {
layoutItems.push( item );
}
}
return layoutItems;
};
```

Start by locating Masonry.prototype._getItemsForLayout and inspect how it filters items before layout. Verify that items with isHidden are excluded while non-hidden items retain their existing behavior; the issue does not name a test file or test command.

Written by the indexing model from the issue text.