Add babel build

Author: developit

.babelrc

@@ -0,0 +1,3 @@
+{
+  "modules": "ignore"
+}

package.json

@@ -3,10 +3,16 @@
   "version": "0.14.0",
   "description": "React's PropTypes, packaged up as a standalone module.",
   "main": "index.js",
+  "scripts": {
+    "build": "babel -f $npm_package_main -s -o $npm_package_main < src/index.js"
+  },
   "repository": {
     "type": "git",
     "url": "https://github.com/developit/proptypes.git"
   },
   "homepage": "https://github.com/developit/proptypes",
-  "license": "BSD"
+  "license": "BSD",
+  "devDependencies": {
+    "babel": "^5.8.29"
+  }
 }