Added two methods to add and remove items.

I've added a feature to allow dynamic insertion of items to a container:
```javascript
var $container = $('#my-container').mosaicflow(),
$new_item = $('<div class="item">A new item</div>');

$container.mosaicflow('add', $new_item);
```
This method keeps original order of items by placing new elements at the end of internal collection.

A removal method was also added:
```javascript
$container.mosaicflow('remove', $new_item);
```

Also, I've modified the way `mosaicflow` gets the height of each item by using `outerHeight()` so now it does not rely on `height` attribute and works with all kind of elements.

Finally, `mosaicflow` was trying to get the `id` attribute to cache its heights, but could fail if items didn't have any. Now it generates one automatically.

Tested in Chrome 26 (OSX), Firefox 20 (OSX) and Opera 12.5 (OSX).