Added commonjs support

Added commonjs support. Old version works as it worked (InputPlaceholder, and TextAreaPlaceholder are global objects)

To use it from other commonjs module: 
```js
var react = require('react');
var initPlaceholder = require('react-input-placeholder');

var input = initPlaceholder(react).input;
// or
var textarea = initPlaceholder(react).textarea;
```