Call error callback on ajax error unless offline

- Expose `Backbone.DualStorage` as a namespace for this plugin's settings
- Treat Ajax status code `0` (instead of every error) to mean we're working offline
- Allow adding more status codes to mean the same using `Backbone.DualStorage.offlineStatusCodes` which is either an array of numbers or a function receiving `response` and returning an array of numbers
- Document `Backbone.DualStorage.offlineStatusCodes`
- Document how `options.dirty` can be examined in the `success` callback to determine whether the operation succeeded remotely (`false`) or only locally (`true`)

(edit: issue #67)