Clean up all DOM nodes when tests end

As the test suite grows it's bad for debugging to leak tons of elements
onto document.body. In order to concisely append/remove on the tests'
"end" event, we need to make sure the root tagName doesn't change. If it
does, the `tree` returned from `app.start` will no longer be the root
node and the removal fn will throw.

package.json

@@ -34,6 +34,7 @@
     "yo-yo": "^1.2.2"
   },
   "devDependencies": {
+    "append-child": "~1.0.0",
     "bankai": "^2.0.2",
     "browserify": "^13.0.1",
     "browserify-istanbul": "^2.0.0",