Give interactions a reference to the map

This is in line with what we do for controls and overlays.  A follow-up change will add calls to `setMap` when interactions are added to a map.
Add addInteraction and removeInteraction methods to the map

This is in line with how we add/remove controls and overlays.
Merge pull request #1173 from tschaub/map-interaction

Give interactions a reference to their map.