feat(broccoli): add diffing MergeTrees plugin

Closes #1815
Closes #2064

tools/broccoli/trees/browser_tree.ts

@@ -3,7 +3,7 @@
 var Funnel = require('broccoli-funnel');
 var flatten = require('broccoli-flatten');
 var htmlReplace = require('../html-replace');
-var mergeTrees = require('broccoli-merge-trees');
+import mergeTrees from '../broccoli-merge-trees';
 var path = require('path');
 var replace = require('broccoli-replace');
 var stew = require('broccoli-stew');