Add explicit index to $matches

Hi,

I've been implementing a custom select drop-down template to provide a search feature for long lists. The existing method in the `bs-select` for retrieving the item doesn't work because the `$index` within the filtered scope changes in relation to the indexes of the `$matches` (I think that's how to describe it).

Anyway, here is my PR to add an explicit index and makes the following code possible:
```html
<input type="text" class="form-control" ng-model="selectFilter" placeholder="Filter..."/>
<li role="presentation" ng-repeat="match in $matches | filter:selectFilter" ng-class="{active: $isActive(match.index)}">
```

PS. this change works with my project but I have no idea how to provide create a unit test to prove this.