Change PropType of className from object to string

Just a simple patch for the type of the `className` property. I also ran `node build.js` to update `standalone.js`.

Steps to reproduce:

When you set the className of the component to a string:

``` jsx
<CodeMirror
  className='editor'
  ...
/>
```

This error would occur:
```
Warning: Invalid prop `className` of type `string` supplied to `<<anonymous>>`, expected `object`. Check the render method of `CodeEditor`.
```