Remove bottom margin of the .well last element

When a *well* contains a paragraph or an element with a margin, this causes an unpleasant effect. We feel that the content of the *well* is not centered.

```html
<div class="well">
  <p>Some text</p>
</div>
```

![napkin 09 08 13 10 48 28 am](https://f.cloud.github.com/assets/611234/937138/7877fb4a-00d0-11e3-8b6f-b7008ae8e6cc.png)

This code sets to `0` the margin of the last element of the `<div class="well">...</div>`.