Allow to access the card index from within the card' template

This should fix issue #6. The card's indices can now be accessed via the card's `$index` property like this:

    <span>{{card.$index}}</span>

The specs were extended to cover this new functionality. The syntax is oriented on the `$index` property inside the child scopes of an `ngRepeat`.

Additionally, the first commit adds two node dependencies that were additionally required to run the tests (two karma plugins).