Adding ListGroup component

Took a stab at #159 (Implement ListGroup).

This solution uses the convenience CSS classes from Bootstrap; so instead of creating `ul` and `li` it wraps everything in a `div` and pushes the `list-group-item` CSS class into all children.  
```
<ListGroup>
  <a href="#link1">To link 1</a>
  <a href="#link2">To link 2</a>
</ListGroup>
```
The above JSX gives;
```
<div class="list-group">
  <a href="#link1" class="list-group-item">To link 1</a>
  <a href="#link2" class="list-group-item">To link 2</a>
</div>
```
  
Also adds support for the props `active`, `disabled` and `bsStyle` to the children, works with ReactBootstrap components as well.
Is this a good solution?
```
<ListGroup>
  <span active>Active Item</span>
  <span bsStyle="success">Success Styled Item</span>
  <span disabled>Disabled Item</span>
</ListGroup>
```
Above JSX gives;
```
<div class="list-group">
  <span class="list-group-item active">Active Item</span>
  <span class="list-group-item list-group-item-success">Success Styled Item</span>
  <span class="list-group-item disabled">Disabled Item</span>
</div>
```

Also matches `activeKey` and `disabledKey` (on parent) against `key` prop on child for another way of adding `active` and `disabled` CSS classes.  

This does not support the `list-group-item-heading` and `list-group-item-text` CSS classes in a good way, maybe a `ListItem` component could be added later to fix this?  

Documentation will be added to this PR depending on feedback (aka; too lazy to create them now if solution is rejected).
Tried out **Atom** (from GitHub) and it really loves adding new-line characters at the end of files.
What code-style should ReactBootstrap use?