feat(directive): add preserveLayout option

When the `preserve-order` attribute is set on the element, images are added by
the order in which they were originally added and not by the time they are
loaded.

E.g.

```
<masonry preserve-order>
    <div class="masonry-brick"></div>
</masonry>
```
Merge pr/24

Close #24
doc(readme): add `preserve-order` docs