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

Sorry for the big pull request but most of the changes rely on each other.

Changes:
* Added new utils functions for working with Reacts `this.props.children`
* Refactored `DropdownButton` to take advantage of the utils children iterator functions.
* Created `Nav` and `NavItem` components
* `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.