Turn on webpack sourcemaps in config file
Build infrastructure for 'update' and 'delete' todo actions

This commit gets things ready for implementing 'update' and 'delete'
actions for the 'todo' resource by adding:
 * constants that represent the names of those actions
 * handlers on the Stores of the 'todo' resource
 * handlers on the dispatcher level for the 'todo' resource

Next steps:
 * Implement a 'TodoItem' component with handler for 'update'
 * Add 'delete' button and handler to 'TodoItem'.
Add TodoItem component with 'update' handler

The TodoItem can directly call the action that updates it's 'todo'
resource, causing a 'change' event to be triggered on the stores, which
triggers a re-render of the parent view TodoApp. For the 'delete'
handler, we don't need to pass the callback from the parent to the child
because the 'change' event will come from the stores, updating the
parent and removing the TodoItem that triggered the change.
Add 'delete' button and handler to TodoItem

As I mentioned in the previous commit, the callback to delete the
'todo' from the stores doesn't need to be passed from the parent
component to affect it - the TodoItem can access the stores directly,
and the 'change' event will trigger the TodoApp to re-render.