Changed key from Math.random() to todo.cid

It doesn’t make much sense to generate a random key for each todo render, because it will re-draw all todo’s DOM nodes on each model change. I changed it to the unique identifier ``todo.cid`` already supplied by the backbone model.
Merge pull request #616 from davidhellsing/patch-1

Backbone example: changed key from Math.random() to todo.cid