asyncComponent = componentPath => {
  const asyncRoutFunc = `(function (location, callback) {
    require.ensure([], (function (require) {
      callback(null, require("${componentPath}"));
    }));
  });`;
  /* eslint-disable no-eval */
  return eval(asyncRoutFunc);
}
Return a function which takes (location, callback). This is compatible with getComponent attribute
of react-router 'Router component'. The function help with webpack to split the main bundle to multiple
chuck which reduces initial load-time.
FIXME: THIS DOESN'T WORK. Need to find a better way - best option now is to upgrade to webpack2 (But it not a trivial change).

@param  {String} componentPath the path of the component in the system. The component is expected to have a default.
@return {function}               the closure function which is used.