Improve commonjs support

Fix #64

Author: vvasilev-

.babelrc

@@ -1,3 +1,8 @@
 {
-	"presets": ["es2015"]
+	"presets": [
+		["es2015", { "loose": true }]
+	],
+	"plugins": [
+		"add-module-exports"
+	]
 }

package.json

@@ -13,7 +13,7 @@
     "png"
   ],
   "scripts": {
-    "test": "./node_modules/.bin/ava --verbose --serial",
+    "test": "./node_modules/.bin/ava test/*.js --verbose --serial",
     "build": "./node_modules/.bin/babel src/ --out-dir lib/",
     "watch": "./node_modules/.bin/babel --watch src/ --out-dir lib/",
     "prepublish": "npm test",
@@ -27,6 +27,7 @@
   "devDependencies": {
     "ava": "^0.15.2",
     "babel-cli": "^6.4.0",
+    "babel-plugin-add-module-exports": "^0.2.1",
     "babel-preset-es2015": "^6.3.13"
   },
   "dependencies": {