Example of binding function as thunk.

Instead of always 'thunkifying' an object, how about using the native bind function? This PR request just adds the following to the `examples`. Maybe it may come in handy as another method of creating thunks without the extra dependency.

```js
var co = require('..'),
	fs = require('fs');

co(function *() {
	var directories = yield fs.readdir.bind(fs, './../');

	directories.forEach(function(directory) {
		console.log(directory);
	});
  
})();
```