Use babel-presets by default

This change makes it so the user doesn't need to specify their plugins.
If they want to override it they would need to modify the js loader
config using the modifyWebpackConfig API. Ideally we would support a
user provided .babelrc as well.

```
exports.modifyWebpackConfig = function(config, env) {
  config.removeLader('js')
  config.loader('js', {
    test: /\.jsx?$/,
    exclude: /node_modules/,
    loader: 'babel',
    query: {
      presets: [
        'babel-preset-react',
        'babel-preset-es2015',
        'babel-preset-stage-0',
      ].map(require.resolve),
      plugins: [
        'babel-plugin-add-module-exports',
        'babel-plugin-transform-decorators-legacy',
      ].map(require.resolve),
    },
  })
  return config;
}
```

Fixes gatsbyjs#235. Requires less user package.json dependencies unless the user
wants to add babel plugins (Users no longer need to provide
babel-plugins of any kind in their project as gatsby provides them)

lib/utils/webpack.config.js

@@ -155,7 +155,14 @@ module.exports = (program, directory, stage, webpackPort = 1500, routes = []) =>
       exclude: /(node_modules|bower_components)/,
       loader: 'babel',
       query: {
-        plugins: ['add-module-exports'],
+        presets: [
+          'babel-preset-react',
+          'babel-preset-es2015',
+          'babel-preset-stage-0',
+        ].map(require.resolve),
+        plugins: [
+          'babel-plugin-add-module-exports',
+        ].map(require.resolve),
       },
     })
     config.loader('coffee', {