issue 681 MarkerLabel creation and MarkerParentModel is gone

This resolves issue #681.

MarkerLabel:
- Any options property of label" " will make the gMarker a MarkerLabel.
- this gets rid of the need for isLabel entirely and the LabelDirective can be deleted.

MarkerParentModel:
- removed so that the main marker logic is all in one place MarkerChildModel.

Before merging I need to remove the directive scope of "=isLabel" and delete the Label directive and models.

@gitnik @cthrax 
comments about the scope issue