Add files entry to package.json. (#26)

Removes unnecessary files from the deployed package.

Deploying unnecessary files impacts the performance of `npm install` for everyone.

See: https://github.com/insin/package-config-checker

package.json

@@ -24,6 +24,9 @@
     "node": ">=0.10.0"
   },
   "main": "./jsonpointer",
+  "files": [
+    "jsonpointer.js"
+  ],
   "scripts": {
     "test": "standard && node test.js"
   },