React 0.12 upgrade

Upgrades all code to use React v0.12.1:
- `react.renderComponent` -> `react.render`
- `react.renderComponentToString` -> `react.renderToString`
- added newly supported html elements, svg elements, and events
- shift internal dart data initialization from `getDefaultProps` to `getInitialState` (`getDefaultProps` is no longer called for every initialized react component)
- wrap react component class in factory

Solves the open issue:
https://github.com/cleandart/react-dart/issues/24