Add ToggleButton component for use as checkmark

Each 'todo' resource has a 'complete' boolean attribute, and we want a
button to toggle this attribute. The idea of a button that acts as a
toggle seems reusable, and so I've added a generic ToggleButton
component.

Next steps are to:
 * Add the infrastructure (handlers on stores and dispatcher) for
updating the 'complete' attribute
 * Use the ToggleButton to add a control to the TodoItem component for
updating the 'complete' attribute of a todo item.