receipts: fall back to localStorage (bug 1021918)

@mstriemer r?
Is there an id in these receipts that can be used in a "dictionary" for faster duplicate checking? This search will keep getting slower as we add more receipts. Also I don't think object equality works the way this code requires:

```javascript
var a = {foo: 'bar'}, b = {foo: 'bar'}
// undefined
a == b
// false
[1, 2, a].indexOf(b)
// -1
```