Allow TabbedArea to control its own state.

Added `initialActiveKey` prop for controlled state.
Determine initial state from first child if not set.
Suppress change events during the `onSelect` callback, then force a
render via setState.
Merge pull request #9 from pieterv/tabbedarea-internal-external-state

Allow TabbedArea to control its own state.
Move to ES6 modules

Export as CommonJS and AMD
Build with Grunt
Fix tests on CI
Add bower.json