Webpack

This should be merged on top of #11.

This is huge PR, sorry :(. But it gets webpack integration and it works great! Though it depends on webpack/extract-text-webpack-plugin#8

- Add webpack loader (works with extract-text-webpack-loader).
- Remove Transformer modules, provide instead `transform(ast, filename)` and
  `transformString(string, filename)` functions.
- Remove CLI utility.
- Some function renamings.
- Provide `styles` prop for React.DOM.* instead of patching `style` behaviour.