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