Module Not Exported for NodeJS

The file micro-enum.js has no `module.exports` definition. Since this file is imported by index.js, the result is an empty object.

Test script:
```js
const Enum = require('simple-enum')

var MyEnum = Enum('A', 'B', 'C')
```

When running that script:
`TypeError: Enum is not a function`

The REPL shows that Enum is an empty object.