Add code coverage script

Author: axic

package.json

@@ -4,6 +4,7 @@
   "description": "Utilities for handling Ethereum keys",
   "main": "index.js",
   "scripts": {
+    "coverage": "istanbul cover _mocha",
     "lint": "standard",
     "prepublish": "npm run lint && npm run test",
     "test": "mocha ./test/**/*.js"
@@ -34,6 +35,7 @@
     "utf8": "^2.1.1"
   },
   "devDependencies": {
+    "istanbul": "^0.4.1",
     "mocha": "^2.3.4",
     "standard": "^5.4.1"
   },