Added removeItemById(id) method

Hi Dan,

First off, this project has been really helpful. Thanks a lot for the time and effort you've put into it.

I implemented this method which does what its name says. I needed it because I wanted a toggling button like this one:

```html
<div ng-hide="inCart()">
    <a class="btn btn-lg btn-primary" ng-click="ngCart.addItem(id, name, price, quantity, data)" ng-transclude</a>
</div>
<div ng-show="inCart()">
    <a class="btn btn-lg btn-danger" ng-click="ngCart.removeItemById(id)" ng-transclude></a>
</div>
```

I saw the comment you left in the code so I decided to give you hand with that.

Javascript is not my strong, so I would really appreciate your feedback if there's something you would've done differently.

Cheers.