add watch task

package.json

@@ -18,6 +18,7 @@
     "nodeunit": "nodeunit",
     "test": "babel-node node_modules/.bin/nodeunit src/**/__tests__/*.test.js",
     "build": "rimraf dist && babel src -d dist --ignore __tests__",
+    "watch": "npm run build -- --watch",
     "postbuild": "npm run doc",
     "prepublish": "npm run test && npm run build && npm run createPackageJson",
     "publish": "cd dist && npm publish"