First attempt at addressing issue #67.

Allows instrumenting the error path with a custom callback to examine the
returned error and decide whether it should be bubbled up (return true)
or handled by Backbone.dualStorage (return false).
Adjust according to feedback on issue #67.

- Rename 'inspector' to 'offline'

- Allow 'offline' to be an array or a function that determines
  if the response should be treated as if the server is offline
  or not. If the server is offline, dualStorage will proceed
  with localsync; otherwise, the error callback will be called.

  An array is expected to contain HTTP status codes. A function
  takes the response object as its argument and return true if
  it should be treated as offline or false otherwise.

- A zero status code is always treated as offline
Some tweaks to make sure we have a valid Backbone.DualStorage object.
Use Backbone.DualStorage now that it's valid.
Use only the global 'offline' function for now.
Document options.dirty in the success callback.
Implement suggestions from @nilbus.

https://github.com/nilbus/Backbone.dualStorage/issues/67#issuecomment-39597301
More suggestions from nilbus.