Create render_react_component controller helper

`render_react_component` allows you to render a React component in place of a typical rails template from `app/views`.

Instead of creating a view that only renders a component, _e.g._
```erb
<!-- app/views/todos/index.html.erb -->
 
<%= react_component "TodoList", {:todos => @todos}, :prerender => true %>
```
You can create call the helper from the controller action instead, _e.g._
```ruby
# app/controllers/todos_controller.rb

class TodosController < ApplicationController
  def index
    @todos = %w{todo1 todo2 todo3}
    render_react_component "TodoList", {:todos => @todos}, :prerender => true
  end
end
```
That's a good idea. I've moved it to another spec in this commit 3073fe6