add pre/postpack scripts for npm5

- in npm5 prepublish is deprecated in favor of prepack
- http://blog.npmjs.org/post/161081169345/v500

Author: alexkrolick

package.json

@@ -18,6 +18,8 @@
   "scripts": {
     "test": "make test",
     "build": "make build",
+    "prepack": "make build",
+    "postpack": "make clean",
     "prepublish": "make build",
     "postpublish": "make clean"
   },