UX Feature Hitting the [Enter] (Return) key creates a new Todo List item

[nelsonic]

Technically there is no other way of adding a todo list item as there are no "submit" buttons.
So we need to have an event listener on the [Enter] key in order for the Todo List to function at all.

Acceptance Criteria

• document.getElementsByClassName('new-todo')[0].addEventListener("keyup" ...
  which creates a new todo item when the [Enter] is pressed.
  see: https://stackoverflow.com/questions/905222/enter-key-press-event-in-javascript
• if the <input class="new-todo"> is empty then ignore the [Enter] keypress.

This issue is part of the TodoMVC Feature List [Epic] #48
Depends on: #57

[nelsonic]