package: add babel compilation.

.babelrc

@@ -0,0 +1,9 @@
+{
+  "presets": ["es2015"],
+  "plugins": [
+    ["transform-runtime", {
+      "polyfill": true,
+      "regenerator": true
+    }]
+  ]
+}

Makefile

@@ -1,5 +1,6 @@
 all:
 	@npm run browserify
+	@npm run uglify
 
 clean:
 	@npm run clean

package.json

@@ -48,6 +48,11 @@
   },
   "devDependencies": {
     "browserify": "13.1.0",
+    "babelify": "7.3.0",
+    "babel-preset-es2015": "6.14.0",
+    "babel-polyfill": "6.13.0",
+    "babel-plugin-transform-runtime": "6.12.0",
+    "babel-plugin-transform-regenerator": "6.14.0",
     "hash.js": "1.0.3",
     "jsdoc": "3.4.0",
     "level-js": "2.2.4",
@@ -69,5 +74,8 @@
     "net": "./browser/empty.js",
     "bcoin-native": "./browser/empty.js",
     "secp256k1": "./browser/empty.js"
+  },
+  "browserify": {
+    "transform": ["babelify"]
   }
 }