Use a reliable source to get the xhr for checking the request status

Backbone.sync only sent xhr as a 3rd argument in Backbone <= 0.9.9.
With Backbone >= 1.0.0, every successful request would fail while
checking the status, as of 863db19f (unreleased).

- Update the Backbone.sync stub to reflect the change in Backbone 1.0.0.
- Update the Backbone.sync stub to return xhr as Backbone.sync does.
- Manually set options.xhr, since Backbone <= 0.9.9 did not set this.
- Prepend _ to _status and _xhr parameters that are not passed in
  Backbone >= 0.9.9. These cannot be used by dualstorage as they do not
  work after 0.9.9 and are just passed along for compatibility with
  these older versions.
Accept options (and callbacks) for syncDirty, syncDestroyed, and syncDirtyAndDestroyed

Closes #79
Changed temporary IDs so they don't conflict with the UUID spec so you can use UUIDs as permanent IDs.
Update CHANGES.md
Give credit for #117
Merge branch 'uuid'