Addresses Issue #616.

React on Rails has been updated so this is no longer needed.

* Update basic generator so that the basic example is simpler
* All files registered for react on rails go in the registration.js file.

If the Rails app that uses this gem doesn't include the
ActionMailer railtie, the gem raises this error:

`uninitialized constant ReactOnRailsHelper::ActionMailer`

This change introduces a check that the ActionMailer
constant is defined before setting the 'inMailer' option.

I'm not quite sure how to test this, so if anyone has
ideas, that would be helpful.

Made the following changes to the node package:

* ComponentRegistry.js: Modified register to detect generator functions
  with three arguments. Set the isRenderer key to true for these
  functions.
* clientStartup.js: Added logic to delegate to renderer functions.
* createReactElement.js: Now accepts an object instead of a component
  name.
* serverRenderReactComponent.js: Throws an error if attempting to
  render a renderer function.
* ReactOnRails.js: Change render function to call createReactElement
  with the component object.

Doc changes:

* README.md: Added section about renderer function under the section
  on generator functions. Moved the section on generator functions
  from the 'ReactOnRails View Helpers API' section to the 'Globally
  Exposing Your React Components' section.
* Added a file code-splitting.md that describes how to use renderer
  functions to do code splitting with server rendering.

Tests:

* ComponentRegistry.test.js: Modified existing test cases to expect
  the isRenderer key to be false. Added a few test cases related to
  renderer functions.
* serverRenderReactComponent.test.js: Show that an error gets thrown
  if trying to server render with a renderer function.
* spec/dummy: Added two examples using rendering functions, one of
  which implements code splitting. Added three test to
  integration_spec.rb.

Resolves: #477

* Slight improvement to the 'no hydrated stores' error
This should make it more clear that rendering
`redux_store_hydration_data` is required.
This should more cleanly differentiate the two cases
* Change hydrated stores error to be a list of actions