More sophisticated RTree implementation

This new implementation is based on
http://github.com/imbcmdth/RTree/, with only a few modifications
to add the optional type and provide the API of the previous
implementation.

There is still room for optimization, but this is such an
improvement over the previous RTree already that it's worth
bringing it in now.

test/spec/ol/structs/rtree.test.js

@@ -13,10 +13,7 @@ describe('ol.structs.RTree', function() {
     rTree.put([-3, -2, -3, -2], 6);
 
     it('stores items', function() {
-      expect(goog.object.getCount(rTree.find([
-        Number.NEGATIVE_INFINITY, Number.POSITIVE_INFINITY,
-        Number.NEGATIVE_INFINITY, Number.POSITIVE_INFINITY
-      ]))).to.be(6);
+      expect(goog.object.getCount(rTree.find([-100, 100, -100, 100]))).to.be(6);
     });
 
     it('filters by rectangle', function() {