Add new icons to create homepage

r? @ngokevin @spasovski

## Before
![screen shot 2014-07-24 at 5 19 11 pm](https://cloud.githubusercontent.com/assets/23885/3695596/9d424e44-1380-11e4-83a4-5bf2df501dfd.png)

## After
![screen shot 2014-07-24 at 5 19 00 pm](https://cloud.githubusercontent.com/assets/23885/3695598/9f23b9b4-1380-11e4-8f8c-b4717a855095.png)

What if:

        &.create-app { background-image: url(../img/icons/item_types/app.svg); }
        &.create-brand { background-image: url(../img/icons/item_types/brand.svg); }
        &.create-collection { background-image: url(../img/icons/item_types/collection.svg); }
        &.create-shelf { background-image: url(../img/icons/item_types/shelf.svg); }