Add prebuild command to delete build dir

Author: Codesleuth

package.json

@@ -5,6 +5,7 @@
   "main": "build/app/app.js",
   "scripts": {
     "postinstall": "cd src && typings install",
+    "prebuild": "rimraf build",
     "build": "tsc -p src/ || true",
     "test": "mocha build/test --require source-map-support/register || true",
     "clean": "rimraf build"