Support locations array as property for geometry layers

Locations default to `content`, but setting `locationsProperty` extends the path.

For example, setting `locationsProperty` to "path" makes the polyline layer pull locations from `content.path` instead of `content`. This works similar to `locationProperty` and the way markers pull their LatLng from the `location` property of their `content`.

Use case: I'm using rectangles in a mapping application to express regions. I've found that it makes more sense for the entire rectangle to be an object with the bounds/locations living as a property of said object:

```javascript
App.region = Ember.Object.create({
  bounds: Ember.A([ ... ]),  // array of L.LatLng
  title: "Region Name",
  otherMetaData: ...
});

App.rectangle = EmberLeaflet.Rectangle.create({
  content: App.region,
  locationsProperty: 'bounds'
});
```

I recognize things could get a little confusing with `locationsProperty` and `locationProperty` coexisting, so I'm open to alternate ideas on how to structure this.