Add update method, remove getters..

This makes the osmNote work a bit more like other osm objects in iD.

- When working with the osm objects, we'll treat them as immutable.
  So all modifications will be through the update method:

  e.g. can do this in a repl, like chrome devtools console:
  >  n = iD.osmNote()
  osmNote { id: -1 }
  > n = n.update({ foo: 'bar' });
  osmNote { foo: "bar", id: -1, v: 1 }

- none of the other osm objects have getters, and in JavaScript all the
  properties are public anyway

modules/osm/note.js

@@ -49,15 +49,7 @@ _extend(osmNote.prototype, {
         return new geoExtent(this.loc);
     },
 
-    getID: function() {
-        return this.id;
-    },
-
-    getType: function() {
-        return this.type;
-    },
-
-    getComments: function() {
-        return this.comments;
+    update: function(attrs) {
+        return osmNote(this, attrs, {v: 1 + (this.v || 0)});
     }
 });