Add React PureComponent and Fragment

[vbfox]

• Add PureComponent doing shallow state & props comparison to avoid render() being called.
• Also add a PureStatelessComponent alias to PureComponent taking no state type parameter. Such components are mapping perfectly to an elm-like architecture with immutable props and no mutable state.
• Add Fragment (class) and fragment helper function to easily return lists from render()

[alfonsogarciacaro]

Awesome, thank you!