Added new utils functions

These functions are useful for working with Reacts `this.props.children`
Refactored `DropdownButton`

This takes advantage of the utils children ilterator functions.
Created `Nav` and `NavItem` component

Implements of functionality of bootstrap Navs
@see http://getbootstrap.com/components/#nav
`TabbedArea` updates

Added correct bootstrap classes.
Refactored to take advantage of new utils children iterators.
Simplified unit tests to only test `TabbedArea` DOM elements.
Created unit tests to test `TabPane` DOM elements.
Merge pull request #12 from pieterv/utils-nav-updates

Added `Nav` and `NavItem` components, refactor of `TabbedArea` to use Nav