Add 'active' prop to NavDropdown component

[mrijke]

This commit adds the 'active' prop to the NavDropdown component, which
when true adds the 'active' class to the component.

Note that this does not automatically fix react-bootstrap/react-router-bootstrap#167, because the LinkContainer does not wrap the entire NavDropdown, but only the individual items.
Personally I fixed this by using a wrapper component wrapping the NavDropdown which checks in the render() if the path of any of its children currently is active (the same check the LinkContainer performs) like so:

  isAnyChildActive() {
    let anyChildActive = false;
    React.Children.forEach(this.props.menuItems, (child) => {
      if (!anyChildActive)
        anyChildActive = this.context.router.isActive(child.props.path, false);
    });
    return anyChildActive;
  }

(in the above code this.props.menuItems is a list of MenuItem wrappers which have the path prop which is passed to the LinkContainer as the to prop)

[taion]

This should go on NavDropdown.

[mrijke]

OK, moved the prop to the NavDropdown component.

[jquense]

LGTM