Enabled adding/removing layers after map creation

[eas604]

This change binds the map's layers to $scope so that they can be added/removed dynamically after map creation and so they can be used in ng-repeat, e.g.:

<esri-map>
    <esri-feature-layer ng-repeat="url in someCollection" url="{{url}}">
    </esri-feature-layer>
</esri-map>

A simple example is at test/add-remove-layers.html.

I know that this module isn't intended to support all JSAPI features, but I thought someone might find this feature useful.

[tomwayson]

Thanks @eas604. I've been thinking that we'd need to add scope to the layer directive to support binding future properties (visibility, opacity, etc) to the controller's scope - so having a working example like this is great! Especially since you've bound it to a collection, that's a good idea. I'd like to review this w/ some others this week, but basically I'm in favor of merging this.

[tomwayson]

@eas604 thanks for your patience w/ this!

[eas604]

No problem. Thanks for merging!