Fix bug when two todo have the same description

[levymoreira]

When create two todo with the same description we have the exception:

 Error: [ngRepeat:dupes] Duplicates in a repeater are not allowed. Use 'track by' expression to specify unique keys. Repeater: todo in todos, Duplicate key: string:
http://errors.angularjs.org/1.2.6/ngRepeat/dupes?p0=todo%20in%20todos&p1=string%3A
    at angular.js:78
    at ngRepeatAction (angular.js:19165)
    at Object.$watchCollectionAction [as fn] (angular.js:11658)
    at Scope.$digest (angular.js:11761)
    at Scope.$apply (angular.js:12012)
    at done (angular.js:7818)
    at completeRequest (angular.js:7991)
    at XMLHttpRequest.xhr.onreadystatechange (angular.js:7947)

This PR solve this problem with base in this article:

https://docs.angularjs.org/error/ngRepeat/dupes?p0=todo%20in%20todos&p1=string:

[addyosmani]

Lgtm