Add backoff file.

I added a little backoff wrapper.

This has a nicer interface:

```js
var Backoff = require('back/backoff');

var b = Backoff(settings);
b.backoff(callback);
b.backoff(callback);
```

Instead of

```js
var Back = require('back');
var extend = require('xtend/immutable');

var b = extend(settings);
Back(callback, b);
Back(callback, b);
```