Add close link to alerts

"Closeable" alerts (ones that the user can close with a click) are useful so I decided to add them in - they are in pure CSS. With that, I needed to remove the need for a `p` element inside the alert `div` a la [Foundation 5](http://foundation.zurb.com/docs/components/alert_boxes.html). So now alerts are created like this:

```html
<div class='alert primary'>Test</div>
<div class='alert success'>Test</div>
<div class='alert danger'>Test</div>
```

and closeable ones like this:

```html
<div class='alert primary closeable'>
  Test
  <a href='#' class='close closeable'>&times;</a>
</div>
<div class='alert success closeable'>
  Test
  <a href='#' class='close closeable'>&times;</a>
</div>
```