Setup Istanbul as the code coverage reporter

http://gotwarlost.github.io/istanbul/

gotwarlost/istanbul#44 (comment)

Author: remarkablemark

package.json

@@ -6,7 +6,8 @@
   "main": "index.js",
   "scripts": {
     "test": "mocha",
-    "lint": "eslint index.js \"lib/**\" \"test/**\""
+    "lint": "eslint index.js \"lib/**\" \"test/**\"",
+    "cover": "istanbul cover _mocha -- -R spec \"test/**/*\""
   },
   "repository": {
     "type": "git",
@@ -28,6 +29,7 @@
   },
   "devDependencies": {
     "eslint": "^3.3.1",
+    "istanbul": "^0.4.5",
     "jsdomify": "^2.1.0",
     "mocha": "^3.0.2",
     "react": "*",