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.~~  
  
~~Also adds support for the props `active`, `disabled` and `bsStyle` to the children, works with ReactBootstrap components as well.~~
~~Is this a good solution?~~  

~~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).
Thoughts on using an array and push each of the `list-group-item-heading` and `list-group-item-text` sides into it, before returning ?

This makes the calls to the `renderHeading` function better; `{this.props.header ? this.renderHeading() : this.props.children}` (As a bonus it also stops adding additional `<span>` elements when `header` isn't used)