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.
Allow interactions to deal directly with browser events

The relayEvent_ method wraps goog's browser events in our MapBrowserEvent.  This allows interactions to deal with real browser events in addition to our emulated events.
Merge pull request #1211 from tschaub/relay

Allow interactions to deal directly with browser events.
Merge pull request #1173 from tschaub/map-interaction

Give interactions a reference to their map.